RIR — reps in reserve
How many more reps you could have done when you ended the set. RIR 2 means you stopped two reps short of failure.
RIR stands for reps in reserve. It's the same scale as RPE, seen from the other side: RIR 0 = RPE 10, RIR 1 = RPE 9, RIR 2 = RPE 8. Many people find reserve easier to think in: "I left two" is clearer than "an eight".
You don't have to go to failure to grow: in the Refalo et al. (2023) meta-analysis, training to failure showed no clear advantage over sets stopped a little short. The working zone for most sets is RIR 1–3: hard enough to grow, without the extra fatigue that eats into your next sets.
The diary handles reserve through RPE: I add it to the reps when I work out your estimated max, but never more than five — beyond that, a guess is too rough.
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In the diary I work out all of these numbers myself — you just log the set.