allowance added to the edge of a pattern piece to give some margin for error, stretch, or repositioning during fitting, the excess to be trimmed away once fixed in place
Trim allowances are particularly common on lining patterns, in order to give a margin for error or adjustment when fitting to the insides of very curved parts of the upper, such as the heel.
Trim allowances are sometimes also used to aid lasting, especially to avoid winkles in the lining. Trim allowance may be left on an upper over the topline of a low shoe so that it can be pulled and perhaps also tacked to the top of the backpart. It is then trimmed away after delasting.