The loop-spec is used in the calculation of correlation functions of atomic positions projected onto the normal modes. Unfortunately, documentation on this is very skimpy, and anyone who is interested will have to dive into the code to understand what is going on.
The loop-spec syntax is as follows:
LOOP-SPEC ::= LOOP [DIAG] [BRIEF] repeat([integer [integer [integer [integer]]]] del) deldel
BRIEF reduces the amount of output showing what calculations will be done.