You are referring to  Procurement Instrument Identifiers (PIID), used to identify all solicitation and contract actions. PIID shall also be used to identify solicitation and contract actions in designated support and reporting systems (e.g., Federal Procurement Data System, System for Award Management), in accordance with regulations, applicable authorities, and agency policies and procedures.)  

A complete list of  PIID's, can be found at FAR 4.1603, along with procedures for use. Refer to FAR Subpart 4.16 for policy on assigning PIID's, as well as identifying both standard and supplementary PIID's.