The string you provided is a designed to test for or exploit vulnerabilities in a database's search or filtering "feature."

: This is used to determine the number of columns being returned by the original query. The attacker adds NULL values until the query stops returning an error, revealing the database structure.

In short, this is not a legitimate feature of a software product, but rather a used to see if a website is properly "sanitizing" the text that users type into search bars or forms.

: This is likely a unique "fingerprint" or tag used by security researchers or automated scanning tools to identify if the injected code was successfully executed in the results.