An authorized user is given a card on someone else's account without being responsible for the debt. The account is then reported on both credit files, which is the part that has consequences.
The account reports in full to both files
Issuers report the account to the bureaus for the primary holder and the authorized user alike, including the limit, the balance and the payment history.
Crucially, history that predates the addition generally comes with it, so a long-established account can appear on a new file as though it had always been there.
That single entry can change average account age, total available credit and payment history simultaneously, which is why the effect is often large.
Liability and reporting are separate questions
The authorized user can transact on the account but has no contractual obligation to repay. The primary holder owes the full balance regardless of who spent it.
Reporting follows the account rather than the liability, which is the asymmetry that makes the arrangement useful for building a thin file.
It also means the authorized user cannot fix a problem they did not cause, since they have no standing to negotiate with the issuer.
The effect runs in both directions
If the primary holder runs the balance up or pays late, that history reports to the authorized user's file as well.
Removal is possible and the entry usually disappears from the user's file, but the damage during the period it was reported may already have influenced a decision.
This is why the arrangement is normally confined to households where both parties can see the account activity.
Lenders discount it when they can detect it
Because the mechanism is well known, it has been sold commercially, with strangers added to seasoned accounts for a fee.
Scoring models and lender underwriting have responded by weighting authorized user accounts less heavily, particularly where the user shares no address with the holder.
A file consisting of nothing but authorized user entries is therefore treated as thin, whatever the score suggests.
It works best as a starting point, not a strategy
The arrangement is genuinely effective for someone with no file at all, because it provides history where none otherwise exists.
Its value falls as the user opens accounts of their own, since a self-established record carries more weight in underwriting.
The sensible pattern is to use it to qualify for a first account and then let that account become the basis of the file.