In the US, practically speaking, how does the distinction between the E2 and E3/E4 subtypes of Class E airspace affect pilots?
"Caution: as noted in the links below, in actual practice, the distinction between E2 airspace and E3/E4 ("extension") airspace is often ignored by ATC..."
In the Center environment, we make no distinction between the surface area and the extensions when dealing with SVFR. The reason is simple. I can't clear a SVFR and an IFR in that airspace at the same time (or two SVFR aircraft), so those extensions should be empty if I'm allowing SVFR. Conversely, I'll not allow a SFVR operation (remember, the pilot has to request it), if I have an IFR aircraft cleared in that surface area.
In the case of Class E that's adjacent to Class C or D airspace, the pilot should be talking to ATC, anyway.
Keep in mind, though, if you're just buzzing along VFR and you see that there's a surface area ahead of you, there's nothing wrong with calling Center and asking if there's known traffic there, or even requesting flight following (cheap insurance!) through the area.
As noted in several answers to this related ASE question, surface-level Class E airspace that actually surrounds the airport for which it is designated-- the airport whose airspace it is intended to protect-- is designated as E2 airspace. Surface-level Class E "extensions" that protect the instrument approaches into other adjoining Class D (or surface-level Class E) airspaces are designated as E4 airspace. E3 airspace is the same as E4 airspace, except that the extension" is adjoined to Class C airspace rather then Class D or Class E "airspace. E3a airspace the same as E4 airspace, except that the "extension" is adjoined to Class B airspace. All this is spelled out in the FAA's "Airspace Designations and Reporting Points" document, current issue FAA Order 7400.11E.
The truth is that the FAA is deeply divided as to whether phrases in the FARs like "surface area", "designated for an airport", and "surface area of controlled airspace designated for an airport" should or should not be construed to include surface-level Class E "extensions", i.e. E3/E3a/E4 airspaces.1 Ambiguity around this issue has existed ever since the 1993 "alphabet" airspace re-designation.2 There's some consideration of a future rule-making effort to address this issue.3 It's possible that the language in AIM 3-2-6, as well as in the legend of the Chart Supplements, that reads "Surface area arrival extensions become part of the surface area and are in effect during the same times as the surface area" will be changed in the future to avoid implying that all regulations that apply to the core "surface area" should automatically be construed to apply to the surface-level Class E "extensions" as well.4
For a list of all the aviation activities that are potentially affected by whether or not phrases like "surface area", "designated for an airport", and "surface area of controlled airspace designated for an airport" are construed to include surface-level Class E "extensions", see the related ASE question What indication has the FAA given that phrases like "surface area of Class E airspace designated for an airport" do or don't include E4 "extensions"?. Among other things, these activities include-- receiving a clearance for Special VFR flight, flying under a ceiling lower than 1000' AGL (without a Special VFR clearance), flying ultralight aircraft, flying unmanned aircraft (model airplanes, "drones", etc), flying unmanned free balloons, and performing aerobatics.
If phrases like phrases like "surface area", "designated for an airport", and "surface area of controlled airspace designated for an airport" are construed to include the surface-level Class E "extensions", then these activities would be prohibited (or in some cases, prohibited without "prior authorization") in the surface-level Class E "extensions".
On the other hand, if phrases like phrases like "surface area", "designated for an airport", and "surface area of controlled airspace designated for an airport" are construed not to include the surface-level Class E "extensions", then these activities would not be prohibited in the surface-level Class E "extensions".
Depending on the exact parsing of the meaning of the words, some of these phrases might be deemed to include the surface-level Class E "extensions" and others might be deemed not to, in which case some of the activities listed above would be allowed in the "extensions" and others would not. It's a bit complicated. Some of the regulations around Special VFR, in particular, do not actually include the phrase "surface area".
To learn more about the rationale behind construing these various phrases to include, or exclude, the surface-level Class E "extensions" (E3/E3/E4 airspaces), see this related ASE question.
Many examples can be found where the FAA is using the phrase "surface area" in a context that is clearly meant to exclude the surface-level Class E "extensions" (E3/E3a/E4 airspaces). The above link explores whether or not that is the most correct understanding of the phrase in terms of interpreting the meaning of various FARs. On the other hand, air traffic controllers often use the phrase "surface area" to mean the entire column of airspace that extends up from the surface in any given location, including any surface-level Class E "extensions" that may be present. There are many examples in various FAA Orders where the word "surface area" is used in a way such that it is not clear which of these two usages is intended.
When thinking about the possible impact of the surface-level Class E "extensions" on various aviation activities, keep in mind that a) Class E airspace has no defined upper limit, except for where it superseded by some overlying higher class of airspace, and b) all of the relevant regulations are written in language that includes phrases like "within the lateral boundaries" of the surface-level Class E airspace in question (which may or include the "extensions", depending on the interpretation we are following), which means that the column of airspace addressed by these regulations has no defined upper limit whatsoever. So construing various regulations to include the "extensions" puts rather large chunks of airspace off-limits to certain aviation activities-- e.g., flying an ultralight.
How to tell E2 airspace from E3/E4 ("extension") airspace on a sectional chart?
Here is an example of E4 ("extension") airspace-- look specifically at the airspace within the dashed magenta line that does NOT surround the airport whose approaches are being protected. That is the E4 ("extension") airspace. The airspace within the dashed magenta or dashed blue line that DOES surround the airport is D airspace (and in this case, reverts to E2 airspace when the tower is closed):
On the other hand, none of the airspace enclosed by the dashed magenta line here is E4 ("extension") airspace. It is all E2 airspace. There is no dashed magenta line that does NOT enclose the airport:
E3 and E3a airspace is depicted like E4 airspace but the dashed magenta "extension" abuts Class C or Class B airspace, respectively, which is depicted differently than the dashed blue or dashed magenta circles in the examples above.
Here is an example of E3 ("extension") airspace--the dashed magenta boundary extending toward the northeast:
Unfortunately there are a few exceptions to this clear distinction between how E2 and E3/E3a/E4 ("extension") airspaces are usually depicted. The ambiguity arises in the case of un-towered airports that have surface-level Class E (E2) airspace. In some cases, long projecting areas of this airspace far from the airport have been included in the E2 airspace description (example: KSHR). In other cases, long projecting areas of this airspace have been designated as E4 airspace (examples: SIT/PASI, BIH, SGU, TVL, and ACV). Of the latter 5 examples, only TVL and ACV have been charted in a way such that the boundary between the E2 airspace and the E4 airspace is actually visible on the sectional chart. The other three are charted just like KSHR, with a single dashed magenta line enclosing all the surface-level Class E airspace. This is explored in more detail, with screenshots, in the related ASE question What is the primary reason that the non-towered airports ACV, TVL, SGU, BIH, and SIT/PASI have E4 extensions to E2 airspace?. Presumably the charting agency will eventually standardize on one of the two methods--and then in the future may need to change the convention, if the FAA definitively determines that there are indeed significant regulatory differences between surface-level Class E "extensions" (E3/E3a/E4 airspaces), and the core surface-level Class E "surface areas" (E2 airspaces) that actually surround the airports for which they are designated.
1,2,3,4) Source-- comments made by high-level FAA staffers during April 2021 airspace charting meeting.
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