AIM 3.2.5 Class D Airspace
Surface area arrival extensions:Class D surface area arrival extensions for instrument approach procedures may be Class D or Class E airspace. As a general rule, if all extensions are 2 miles or less, they remain part of the Class D surface area. However, if any one extension is greater than 2 miles, then all extensions will be Class E airspace.
Surface area arrival extensions are effective during the published times of the surface area. For part–time Class D surface areas that revert to Class E airspace, the arrival extensions will remain in effect as Class E airspace. For part–time Class D surface areas that change to Class G airspace, the arrival extensions will become Class G at the same time.
e. Functions of Class E Airspace.
Extension to a surface area. Class E airspace may be designated as extensions to Class B, Class C, Class D, and Class E surface areas. Class E airspace extensions begin at the surface and extend up to the overlying controlled airspace. The extensions provide controlled airspace to contain standard instrument approach procedures without imposing a communications requirement on pilots operating under VFR. Surface area arrival extensions become part of the surface area and are in effect during the same times as the surface area.
3. Airspace used for transition.
Class E airspace areas may be designated for transitioning aircraft to/from the terminal or en route environment.
Class E transition areas extend upward from either 700 feet AGL (shown as magenta vignette on sectional charts) or 1,200 feet AGL (blue vignette) and are designated for airports with an approved instrument procedure.
The 700-foot/1200-foot AGL Class E airspace transition areas remain in effect continuously, regardless of airport operating hours or surface area status.
Source Aim 3.2.5