Documentaton - BLOCKS - BKSPLIT

[REDUCE, MAPKEYS] = BKSPLIT(BK, KEYS) Splits KEYS into N chunks, where N is specified by BK.split. Runs N processes via SSH, which continue at this line of the file with REDUCE = 0 and MAPKEYS equal to the split set of keys. Upon finishing, REDUCE will be set to 1, and MAPKEYS will be empty (since work has been completed).

BKSPLIT(...,'MAP',MAPKEYS) is called internally, to signify we have already mapped the keys and perform some action on them.