Glossary
Walking a seam
Walking a seam is stepping one pattern piece along another, a short distance at a time, to check that the two edges are the same length and that the notches meet.
It is done with the pieces themselves rather than with a tape, because most garment seams are curved and a curve cannot be measured accurately in a straight line. You pivot one piece against the other and follow the edge round.
Walking the armhole against the sleeve cap is the check that decides whether a sleeve will set in. The cap should be longer than the armhole by the intended ease and by nothing else.
This is the step professionals still do by hand after grading, on every size, because software has historically told them the geometry was fine when it was not. Automating it credibly means measuring the curves and showing the numbers, not asserting that it passed.