Glossary
Notch
A notch is a small mark cut into the edge of a pattern piece that tells you which point on one seam meets which point on another.
Two seams of the same length can still be sewn together wrongly if the ease is distributed in the wrong place. Notches are what prevent that: they are the registration marks of a garment.
Conventionally, one notch marks the front of a sleeve cap and two mark the back, so a sleeve cannot be set in backwards. That convention is not universal, which is one of the small reasons files move badly between workrooms.
Missing notches are one of the most common defects in a pattern file that has been converted between systems — the geometry survives the trip and the registration marks do not.