What it is
The identifier the state attaches to a license record. It is the key that turns “a company said it is licensed” into something you can look up yourself in about a minute.
Where it appears
Commonly on estimates, contracts, invoices, business cards, vehicle lettering and yard signs. Every listing on this site shows the number from the state's record.
If a business will not give you a number, that is your answer.
How to check it
Search this site by name, or use the state's own lookup. Both read the same register: ours is faster to search, theirs is authoritative and always current.
Where this comes from
The department's copy is the live one. Ours is a dated snapshot, and the date is in the footer.