AGL — Above Ground Level. Height above the terrain directly below. Used for Part 107’s
400 ft ceiling and most airspace ceilings of Class C, D, E (700/1200 AGL).
MSL — Mean Sea Level. Absolute elevation above the ocean. Used for Class A (FL 180
MSL – FL 600 MSL) and many sectional-chart ceiling callouts.
Pilot trap: the same “100 ft” means very different things depending on which frame.
If a sectional ceiling says “70” without parentheses, it’s MSL (add 00 → 7 000 ft MSL).
If it’s in parentheses like “(12)” it’s AGL.