Which PIID is used to identify amendments to solicitations and modifications to contracts?

The selection of the supplementary PIID, which stands for Procurement Instrument Identifier, is significant because it is specifically designated for tracking amendments to solicitations and modifications to existing contracts. In procurement processes, it's essential to have a unique identifier that remains linked to the original solicitation or contract while allowing for any amendments or changes to be tracked efficiently.

This helps maintain clarity and historical context, ensuring that all parties involved can easily identify the evolution of the contract or solicitation without confusion. The supplementary identifier is vital for auditing, compliance, and administrative purposes, as it provides a systematic approach to managing changes throughout the lifecycle of the procurement process.

In contrast, the primary PIID is used to identify the initial contract or solicitation, while the other identifiers like secondary and auxiliary do not specifically address amendments or modifications, which is why they are not the correct choices in this scenario. The supplementary PIID serves this specific function, making it the right answer.