I think it just stands for Packet ID. From my very limited experience I believe that it has do with a code signal that provides information that allows a receiver to separate the different media signals coming from a single satellite. There is a VPID for video and an APID for audio. I am sure this explanation is probably flawed. I only know what I know from researching Free To Air satellite TV. Hooray for Free Satellite TV! There was one site that explained it, but I can't remember the site or what they said. But, I did find some info here: http://www.satelliteguys.us/free-air-fta-discussion/106432-thelist.html You could just post this question on that forum and I am sure one of the satellite guys would help.
http://satelliteguys.us/free-air-fta-discussion/ I hope this helps :)