Specifically, 1st substitute means 'repeat the 1st flag in the hoist' etc. To show 'AABB' you would hoist A, 1st sub, B, 3rd sub.
You can therefore show any combination of letters in groups of up to 4, beyond that some combinations aren't possible so signals like '**** OFF' can't be flown. International code signals are all ways groups of 1, 2, 3 or 4 flags.
If you were a club race officer you would fly the 1st sub on its own for 'general recall'