In plain terms
It is issued when the agency approves the data broker account, and it stays with the registered business. It is not the API key and it is not a secret in the way a key is: it identifies the registrant rather than authorizing access. You will see it in the portal, in file naming, and anywhere your own records need to point at the registration they belong to. One business, one account, one ID. DROP does not support several logins on a single broker account, though a broker can add an authorized agent as a contact.
What it means for a registered broker
Store it with your registration record rather than in somebody's inbox, next to the confirmation number and the date you filed. Those three together are what you would produce to show the registration exists. If the business restructures or the filing entity changes name, sort the registration out first, because the ID follows the registered entity and everything else hangs off it: the key, the list selection, and every status file you have submitted under it.