The descriptor management services provide interfaces for allocating, freeing, creating, locking and unlocking protected mode descriptors in the descriptor table, which can be either the Global Descriptor table (GDT) or the current task's Local Descriptor Table (LDT).
In general, these functions operate on the LDT when running in a DPMI environment, such as a Windows/xx DOS box, or under Linux's DOSEMU. Normally, when running in real DOS, these functions will operate on the GDT.
Because 32-bit programs don't need descriptors to address all of memory in the way that 16-bit programs do, your 32-bit programs will not generally use these functions. However, if your 32-bit program needs to call real-mode code (which is always 16-bit), or if your program contains both 16-bit and 32-bit code, you will need these functions.