A unique identifier refers to a special code or number that helps identify something or someone uniquely, making it different from everything else. It helps distinguish one particular item or person from all others. Full definition
Note: If collecting samples from multiple dogs, make sure to mark the collection cards with unique identifiers for each dog prior to blood collection to avoid sample mix - up. (pawprintgenetics.com)
The apostille number becomes a kind of unique identifier of the document, referring back to a secure database by which the issuance can be confirmed. (slaw.ca)
What Verizon did there was stick its customers with unique identifiers on a network level. (businessinsider.com)