AGL is height above the ground below the aircraft. MSL is height above mean sea level. On a sectional obstruction the bold number is feet MSL and the number in parentheses is feet AGL, and the Part 107 altitude limit is AGL.
What each abbreviation measures
Above ground level starts at the surface under the aircraft. If the ground there is a riverbank, a roof, or a ridge, AGL is the vertical distance from that surface up to the aircraft. Mean sea level starts at the average level of the sea. A field that sits in the mountains can be thousands of feet MSL while an aircraft 200 feet over the runway is only 200 feet AGL.
AGL is not always a larger number than MSL. At a coastal site the two can be close. At a high airport they are far apart. MSL is not a night-only datum. Both references are used in daylight and at night. The test item that asks how the abbreviations differ is answered by the definitions themselves: AGL is height above the ground below, and MSL is height above mean sea level.
Section 107.51(b) is an AGL rule. The small unmanned aircraft cannot be higher than 400 feet above ground level unless it is flown within a 400-foot radius of a structure and does not fly higher than 400 feet above the structure's immediate uppermost limit. A shelf printed in MSL does not reset that limit. Class G does not reset it either. The full altitude rule is on the 400-foot page.
Where the chart prints each one
Obstruction symbols carry both. The bold top number is the elevation of the top in feet MSL. The number in parentheses is the height of that obstruction in feet AGL. Reading them backward is how a tower inspection gets planned at the wrong altitude. The pair is covered again on the obstruction heights page. This page is where the structure exception is computed.
Class B shelf labels are MSL. The Chart Users' Guide prints the ceiling and floor of each sector in solid blue figures with the last two zeros omitted. A label of 80 over 40 means 8,000 feet MSL over 4,000 feet MSL. Class C uses the same MSL idea in magenta figures, again with the last two zeros left off. A plus sign before a Class B floor means the floor is upward from above that altitude, so flight at and below the printed floor is outside that shelf. Those labels are not AGL, and they are not a Part 107 ceiling.
Field elevation in an airport data block is feet above mean sea level. It is not feet AGL, and it is not a distance in nautical miles. The elevation tells you how high the airport sits relative to the sea. It does not tell you how high you may fly. Quadrant maximum elevation figures are also MSL, and they are not a clearance to climb. How those figures are built is on the MEF page.
A tower top, using the printed AGL height
An obstruction is labeled 2,341 with (387) underneath. You will inspect it and stay within 400 feet horizontally of the structure. The bold 2,341 is the top of the obstruction in feet MSL. The 387 in parentheses is the height of the structure above the ground, in feet AGL. Section 107.51 allows flight above 400 feet AGL only when you remain within 400 feet of the structure and you stay no more than 400 feet above its immediate uppermost limit.
Start from the AGL figure. 387 feet AGL plus 400 feet is 787 feet AGL. That is the top of the altitude exception. The MSL number is there so you can picture the top against sea level. It is not already an AGL height, so you do not add 400 to 2,341 and call the result 2,741 feet AGL. You also do not stop at 400 feet AGL and ignore the exception once you are inside the 400-foot horizontal radius. The parenthetical height is the one the exception stacks on.
If the parentheses were missing, you still would not invent an AGL height by subtracting the airport's field elevation from 2,341 unless the tower stands on that same elevation. The chart prints the AGL height so you do not have to guess the ground under the tower.
The swap the questions are written to catch
Three misses show up together. One treats MSL as a night rule, or assumes AGL is always the higher of the two numbers. One reads field elevation as feet AGL. One treats the bold obstruction number as AGL and either adds 400 to it or refuses to add 400 to the parenthetical height. The legal altitude for the inspection above is 787 feet AGL, reached by adding 400 to 387, while 2,341 remains the MSL elevation of the top.
Practice the same split on the free practice test. The chart lessons that drill it are in the course.