Make a deal meet conditions (fields filled, right role, an approval, a time limit) before it can move into a stage.
A blueprint is a gate on one stage of your deal pipeline. It is checked when someone tries to move a deal INTO that stage, whether by dragging the card on the Deals board or by editing the deal. If the deal does not meet the gate, the move is refused and the deal stays where it was. Admins can always satisfy the role part of a gate.
Blueprints apply to deals. They do not apply to estimates, proposals, engagements, tickets or leads: the stage of an estimate or proposal is simply its document status, and engagements have their own stages. If you sell with quotes and have no deals, you do not need blueprints.