When the system was running low on file space (which was pretty frequent given the cost of drum and disc memory), this unload process would begin. First, files which had not been modified would be selected to be marked as unloaded, then the actual data blocks released for use. If things really got nasty, a ROLOUT run would start which would write out files with no current backup, and then mark them as unloaded.
Referencing an unloaded file would result in it be automatically restored when it was assigned to your run.
All this in the 1970's. Amazing, eh?