Michael Hall has already given you the correct answer. I just want to add clarification.
The Mode C Veil is the thin magenta line surrounding the Class B airport. The line is always depicted as thinner Than the border of Class C airspace. In fact, the two have nothing to do with each other. The Mode C Veil is the outer limit at which a Mode C compliant transponder must be operational regardless of airspace (except for certain aircraft). As of 2020, this regulation also applies to ADS-b Out. By regulations, it is a circle around the center of the controlling Class B airspace with a radius of 30 Nautical Miles per 14 CFR Appendix D Part 91
Appendix D to Part 91 Airports / Locations: Special Operating
- Locations at which the requirements of §91.215(b)(2) and §91.225(d)(2) apply. The requirements of §91.215(b)(2) and
§91.225(d)(2) apply below 10,000 feet MSL within a 30-nautical-mileradius of each location in the following list.
SeaTac airport is on this list.
The border of Class C airspace is denoted by a thicker solid magenta line where as the border of a TRSA (baby C) is denoted by a thicker solid gray line. The shape of Class C airspace is generally like an upside down wedding cake. The lower level starts at the surface and extends upward to 1200 feet AGL with a radius of 5 Nautical Miles. The upper level starts at 1200 feet AGL and extends upward to 4000 feet AGL with a radius of 10 Nautical Miles.
The border of Class D airspace is denoted by a cyan blue segmented line forming a circle of generally 4 Nautical mile radius from the center of the controlling airport. The dimensions of Class D airspace generally start at the surface and extend upward to 2500 feet AGL unless otherwise noted.
The border of Class E airspace is denoted in several different ways depending on the type of Class E airspace. Two of which are depicted in your example of SeaTac Airport even though you easily see only one. These are
- Class E to the Surface - thin segmented magenta line.
- Class E surface Extension - thin segmented magenta line.
- Class E Transition airspace starting at 700 AGL - shaded magenta line.
- Victor and other federal airways - pale lines generally blue in color.
- Offshore Class E starting at various altitudes - blue zipper lines.
- Domestic enroute Class E - shaded blue lines when starting at 1200 feet AGL and zipper lines when at different altitudes.
- Class E Above 60,000 feet - usually not denoted.
Class G is not denoted. It can be generally thought of as anything below, above and outside of all other airspace. On a US sectional chart, it generally Class G starts at the surface and extends upward to the Class E Domestic enroute altitude of 1200 feet AGL unless the surface elevation is greater than 13,000 feet MSL. Then, Class G extends upward to 1500 feet AGL. Under the Class E Transition area, Class G starts at the surface and extends upward to 700 feet AGL. It can also be located on the darker solid side of the shaded blue line of Class E Domestic Enroute starting at the surface and extending upward to 14,500 feet MSL or 1500 feet AGL. Whichever is higher.