It's amazing how old ZPL is. I wrote a Zebra-140 driver (in C..) in something like 1993.
https://github.com/jhallen/joes-sandbox/tree/master/doc/zebr...
It's a simple macro-based typesetting language that can be used to quickly make many different label images from a database. You uploaded the compiled labels into a handheld computer so that labels could be printed on a production line. One option for the computer was the Psion Organizer II.