Tags, minimums and short ends
The rack is the list of offcuts your shop keeps: lengths of bar and pieces of plate, each with a tag such as R-0007 or P-0002 to write on the metal with a marker.
A leftover at least as long as your minimum offcut (300 mm unless you change it) goes back on the rack when you mark a plan as cut. Shorter leftovers are short ends; the app does not track them.
Plate leftovers come back when both sides reach your minimum plate side (150 mm unless you change it).
You can add an offcut by hand, shorten one after a cut made away from the app, or remove one. Every removal can be undone right after.