What does 0 RIR mean?

Zero RIR means no additional clean repetition was available. One RIR means roughly one rep remained; three RIR means roughly three remained.

Why does RIR matter?

Compare 225×5 performed with 4 RIR to 225×5 performed with 0 RIR. The log looks identical if it records only load and reps, but the second set was dramatically harder. Effort data helps distinguish a comfortable prescription from one already near the lifter's limit.

Do you need to train at 0 RIR?

No. Training to failure is not required on every set. Productive work can be performed with repetitions in reserve, especially when fatigue management and repeated high-quality work matter.

How accurate is RIR?

It is an estimate, and accuracy generally improves with experience. Treat it as a useful signal alongside load, reps, technique and training history.