Only appears on sets your routine has configured to ask for it — right after you confirm reps, before the set is logged.
The value starts on "--" (not recorded) unless you already entered one for this set. It's purely for your own reference; the coaching engine never reads it and it never blocks logging.

Nothing about this screen can go "wrong" — it's a note to yourself, not a number the app grades you on.
Related: "What is RIR (reps in reserve)?", "The reps screen on your watch".