This system variable is automatically set to contain the remainder of the command line used to start the current program. For example, if the program was started with:

RUN FALALA do re mi

Then A$ = CMDLIN would set A$ to "DO RE MI" (always folded to upper case).

xcall LSTLIN, A$ on the other hand, would return "RUN FALALA do re mi" (the complete command line with no folding).