LOCAL_C, LOCAL_D, GLREF_C, GLREF_D, GLDEF_C and GLDEF_D are all defined in e32def.h as:

The _C and _D suffixes are a convention. _D indicates a data declaration, and _C a code one.

So LOCAL_D is used to indicate a static data while LOCAL_C indicates static code.

By contrast, GLREF_C and GLREF_D, by contrast, are used to indicated GLobal REFerences (i.e. extern), and GLDEF_C and GLDEF_D indicate GLobal DEFinitions. These terms don’t have a formal definition but are used as markers for functions or variables which have been declared as GLREF_C or GLREF_D respectively elsewhere.