IDs, Ref. No, PO Number, Doc.No

I noticed that on different transaction forms, there are reference to IDs in different format that won't allow me to customise.

Eg: In Transactions > Purchase Orders >  PO # is auto generated by Netsuite (I think) - I can't find that in any custom forms.

When a new Purchase order gets created, it shows "PO #  To Be Generated  "

Eg: In Cutomize Sublist >  There is something called PO/Check Number.
In additional filter, there are more numbers...
Number / ID, PO/Check number..

Somewhere else, I have seen Doc.No. and Ref. No.

Basically, I want to be able to customies Bill Credit and Item Receipt to include a Purchase Order number so that I can match up the record with the original Purchase order and be able to trace records easily.