To accomplish compression and expansion, the macro uses two dictionaries: the compression dictionary and the expansion dictionary. These dictionaries are related logically and physically. When you expand data that has been compressed, you want the result to match the original data. Thus the dictionaries are complementary. When compression is done, the expansion dictionary must immediately follow the compression dictionary, because the compression algorithm examines entries in the expansion dictionary.
The value of CMPSC_SYMSIZE represents the size of the compression and expansion dictionaries. For example, if CMPSC_SYMSIZE is 512, then the size of the compression dictionary is 512 and the size of the expansion dictionary is 512.