Other than what you mention above about BPA call numbers, I did find this DPAP website, http://www.acq.osd.mil/dpap/dars/order_code_assignments.html
, which may answer your question. The website refers you to DFARS 204-7005
, Assignment of order codes. Below is a copy of DFARS 204-7005
(a) Defense Procurement and Acquisition Policy, Program Development and Implementation, maintains the order code assignments for use in the first two positions of an order number when an activity places an order against another activity’s contract or agreement (see 204.7004(d)(2)).
(b) Contracting activities shall follow the procedures at PGI 204.7005 for requests for assignment of or changes in two-character order codes