48 C.F.R. § 4.001
As used in this part-
Procurement Instrument Identifier (PIID) means the Government-unique identifier for each solicitation, contract, agreement, or order. For example, an agency may use as its PIID for procurement actions, such as delivery and task orders or basic ordering agreements, the order or agreement number in conjunction with the contract number (see 4.1602).
Supplementary procurement instrument identifier means the non-unique identifier for a procurement action that is used in conjunction with the Government-unique identifier. For example, an agency may use as its PIID for an amended solicitation, the Government-unique identifier for a solicitation number (e.g., N0002309R0009) in conjunction with a non-unique amendment number (e.g., 0001). The non-unique amendment number represents the supplementary PIID.
48 C.F.R. §4.001