CUSIP
9-character security identifier for US and Canadian equities, bonds and options.
CUSIP stands for "Committee on Uniform Securities Identification Procedures" and is the standard identifier for securities traded in the United States and Canada. The first six characters identify the issuer, the next two the specific security, and the last position is a check digit.
In 13F filings the CUSIP is the unique link between a holdings row and a real-world security. Because tickers change, mergers happen and class structures (Class A vs. Class B) exist, the CUSIP is the only reliable key for cleanly comparing positions across quarters.
International securities use the ISIN (International Securities Identification Number) instead, which embeds the CUSIP in its US variant.