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.