Step 1: Think about how a pattern leaves the mould.
After the sand has been rammed firmly around a pattern, the pattern has to be lifted straight out to leave a clean cavity behind, and this withdrawal happens along one particular direction, usually straight up.
Step 2: Consider what happens to surfaces that run parallel to that withdrawal direction.
Any surface of the pattern that is parallel to the direction of withdrawal, that is, any vertical surface, stays in continuous rubbing contact with the surrounding sand for the entire length of the pull, and this friction can drag sand grains loose, damaging the mould cavity edges. Horizontal surfaces, being perpendicular to the pull direction, are not scraped in this way and do not need any such adjustment.
Step 3: See how draft solves the problem.
By tapering these vertical surfaces very slightly, giving them what is called draft or taper allowance, the contact area with the sand reduces sharply as soon as the pattern starts to move, letting it slide out cleanly without tearing the mould. This is why draft allowance is applied specifically to the vertical surfaces of a pattern.
\[ \boxed{\text{vertical surfaces}} \]