When we overcall the Opponent's opening bid with 1NT, that shows 15-18 HCP with a stopper, and "systems on".

While that is adequate, one danger which is much greater is that we might get doubled. So we define good runouts:

Opponents Double our 1NT Overcall

In the discussion that follows, we assume the Opponent's suit is ♠, and =low unbid suit, =middle unbid suit, =high unbid suit. But the system works for any three unbid suits.

After Pass forcing Redouble

Assuming the Opponents have been quiet so far, Advancer's choices are: