This tells the compiler to chang the pprefix in the cbk to inv-.
Another user could use sinv- for his/her purposes. It can also be used for both input and output files to make the field names unique.
I usually include the "-" between the colons so that I can elliminate the prefix if I have a mind to. I.e. ==:pre-:== by ====. You may need to separate those 4 =s w/a space. In fact check all the syntax here. I don't have a manual handy and I'm quoting from memory.