Class D airspace (the blue dotted circle) always starts at the surface. The ceiling of that airspace is indicated by a number in brackets. If you see  as an example, that indicates that we have Class D controlled airspace from the surface up to 3,500 ft. MSL.
But what if you see a negative number in the brackets?
If you see , that means you have Class D airspace starting at the surface, with a ceiling of 3,500 ft. MSL. Meaning that at exactly 3,500 ft., it’s still Class D airspace.
But if you see [-30], that indicates Class D airspace starting at the surface up to (but not including) 3,000 ft MSL. So at exactly 3,000 ft. MSL, the airspace will change to the overlying airspace.
So why the trickery here?
Well, it has to do with overlying airspace and whether or not 3,000 ft. would need to be classified as Class D or the floor of the next airspace. It is one of those quirky FAA nuances that you won’t likely need to know for your test, just an interesting tidbit.