It's your unique job number for your establishment manning table -the 8005. For example your Coy, Sqn, Bty establishment of say 103 pax will be split down into roles, from OC down to clerk and a 6 figure (off the top of my head) number given to it - your PID.
Just realised I'm being serious and all grown up. To that end - tongue my date.
I just love ARRSE - in 10 replies we have three versions of what a PID is - take your pick - Per Indent, Postion ID or Personal Identification number...trust no-one!!! Just goes to prove that every ****** has an opinion on here and most of them are wrong!