"Op" is short for "Operation", which is individual steps in the overall process. Numbers are identifiers of each step. Common practice is to list individual steps in increments of 10 so that new steps can be inserted in the process without having to re-do all of the existing process documentation.
TygerDawg
Blue Technik LLC
Manufacturing Engineering Consulting
They can use those designations for the different operations or even tools used during the machining process. Like op 1 maybe a face mill to establish the initial surface, then Op2 could be a drill to remove material for a pocket or series of pockets in that face, then Op3 could be a roughing cutter to rough out the pockets, Op 4 a finishing cutter to finish the pocket. This should give an idea of what its meaning is.
The history? Its just a logical way for the operator to know where they are at with in the program, especially if they have to change a dull tool, and search for that operation.
