On a sectional obstruction symbol, the bold number is the height of the top in feet above mean sea level, and the number in parentheses is the height above ground level. A symbol that shows 1,230 in bold and (825) underneath is 1,230 feet MSL and 825 feet AGL. Both figures describe the same structure.
How the symbol is drawn
The Chart Users' Guide prints specific obstacles wherever the chart has room. In a dense city, only the highest obstacle in a group may be shown, using the group-obstacle symbol, so the chart stays readable. When space allows, the AGL height is shown in parentheses under the MSL elevation. The FAA sometimes omits the AGL value where printing both would be confusing. If you see only one number, do not invent the missing one by guessing.
Letters and extra marks change the meaning of the symbol without changing the two heights. UC next to the obstacle type means the obstacle is under construction. High-intensity lights may operate part time or by proximity, and the chart notes that lighting pattern separately. Guy wires can extend outward from a tower even though the height numbers describe the structure itself. A wind-turbine farm is a hatched area. The boxed elevation inside it is the MSL elevation of the highest turbine. Turbine lights sit on the nacelle, and the blade tips can be higher than those lights. None of those notes swaps MSL and AGL.
The parenthetical value is not the airport elevation. It is not a surveyed duplicate of the bold number in the same datum. The bold value is not feet AGL, and it is not nautical miles. If a choice says both numbers are MSL, or that 1,230 is height above the airport while 825 is above sea level, the datums have been reversed.
One tower, two references
Take the symbol that prints 1,230 over (825). The top of the obstruction is 1,230 feet above mean sea level. The structure rises 825 feet above the ground at its base. You can see that the ground under the tower is lower than the airport might be, because 1,230 minus 825 is the elevation of that ground, but the test does not ask you to subtract. It asks you to name each number. Bold is MSL. Parentheses are AGL.
Do not add 1,230 and 825 to manufacture a third height. They are not stacked. The MSL figure already includes the ground elevation plus the structure. The AGL figure is only the structure. Adding them double-counts the tower.
The quadrant's Maximum Elevation Figure is a different number again. It is a rounded MSL figure for the whole 30-minute box, built by the procedure on the MEF page. It is not a substitute for the two numbers on this symbol, and this symbol is not a substitute for the MEF. AGL versus MSL is the datum page. This page is the symbol.
What the heights are for under Part 107
Section 107.51(b) limits the aircraft to 400 feet AGL, with one structure exception. Within 400 feet horizontally of a structure, the aircraft may fly up to 400 feet above that structure's immediate uppermost limit. The AGL height in parentheses is the height of the structure above the ground. The bold number is the sea-level elevation of the same top. When you apply the exception, you use the AGL height of the structure, not the MSL figure, as the starting point. The arithmetic for a labeled tower is worked on the AGL page from the obstruction in that question. Do not build a second total from 825 here and treat it as a different rule.
The 400-foot horizontal distance is measured from the structure. Being merely in the same quadrant as a tall tower does not put you inside the exception. The 400-foot rule states the limit when no structure is close enough to use.
The trap
The questions are written to swap the parentheses and the bold type. Parentheses are AGL. Bold is MSL. On 1,230 and (825), the answers are 1,230 feet MSL and 825 feet AGL. Treating both as MSL, calling the bold number AGL, or deciding that 825 is the sea-level height are the three ways to miss a symbol you can read in one glance. UC, lighting notes, and a group symbol do not change which number is which.
Read obstruction items on the free practice test. The chart lessons are in the course.