The record that an instance has been set up, and by whom.

A row here is the claim, and there can be at most one. This is a table of Ithibati's rather than a flag on the account for two reasons, and the second one decides it.

The application would have to create the flag itself, on a table Ithibati does not own, together with a partial unique index that is the entire guarantee. Forgetting that index fails nothing until two people register at the same moment. Here Ithibati.Migration.up/1 creates the index, so nobody can forget it.

A boolean on an account also conflates two different facts: this instance has been set up and this account set it up. Delete that account and both vanish, which makes a second setup possible again. user_id is nullable and nilified on delete, so the row goes on saying that the instance is set up even when the account that set it up is gone.