Water heater expansion tank

What is the difference between a well expansion tank and a water heater/boiler thermal expansion tank?
Had an inspection where a 2 gallon H20-TO brand (maroon colored that are usually used for well systems) expansion tank was installed on the water heater. Usually they say thermal expansion tank, whereas this one did not (mod #T2; Date code - 14810765).

Can someone please enlighten me.