Glossary
Truing
Truing is correcting a pattern after it has been altered so the seams that must match still match and the lines that cross a seam still run smoothly through it.
Move a dart, add fullness or lower an armhole and the edges around the change stop agreeing with their neighbours. Truing is the pass that fixes that: lengths walked back into agreement, corners squared to the seam, curves blended so a hem crosses a side seam without a kink.
It is unglamorous, it is where a great many errors are introduced and caught, and it is almost entirely mechanical — which is exactly the kind of work that should be checked by the file rather than by eye at the end of a long day.