When airspaces adjoin vertically, which airspace class does the shared flight level or altitude belong to?

Airspaces have a lateral and vertical boundary, which results in airspaces adjoining other and thus having a shared vertical boundary. Which airspace class do these shared vertical boundaries belong to?

Example: Dortmund's (EDLW) control zone is a class D from GND/SFC to 2500ft MSL, on top of this control zone is a TMA airspace class D from 2500ft MSL to 4500ft MSL thereafter class E from 4500ft MSL to FL100and above that we have class C airspace from FL100 to FL660.

(Image Source: www.skyvector.com - Map Data: skyvector, ARINC, OpenStreetMap)

If an aircraft wants to occupy 2500ft MSL, 4500ft MSL or FL100, which airspace class are they in?

According to ICAO Annex 11 - ATS (Air Traffic Services)PDF, the less restrictive airspace is in effect on shared boundaries:

Note.— Where the ATS airspaces adjoin vertically, i.e. one above the other, flights at a common level would comply with requirements of, and be given services applicable to, the less restrictive class of airspace. In applying these criteria, Class B airspace is therefore considered less restrictive than Class A airspace; Class C airspace less restrictive than Class B airspace, etc.

Unless national regulations in the country's AIP are in contradiction or the national AIP does not list any information on this, the ICAO regulation will apply.

In the above example, the altitudes or flight level would be considered:

2500ft MSL: Class D airspace
4500ft MSL: Class E airspace, since class E is less restrictive than class D
FL100: Class E airspace, since Class E is less restrictive than class C

• One exception to this is the boundaries of U.S. class A airspace, where aircraft at 18,000 feet AMSL (the lower boundary with class E airspace) or at FL600 (the upper boundary with class E airspace) fall in class A airspace, despite class A being more restrictive than class E. EDIT: I realise that the original question was tagged "easa-regulations"; however, SentryRaven's answer cites general ICAO rules, which is what this comment is responding to. – Vikki - formerly Sean May 10 '18 at 18:29