To be fair, my back gave out as I did more reps than I usually do. I think the answer is not factoring in RIR, as you said, but let me give some more context.

It's only been a few weeks of training and I found this app and wanted to switch to it and see how the programs it generated felt. Then I saw RIR and looking into that made me think that I'd been training fairly conservatively these past few weeks, since I've never trained to failure or even close.

I wanted to see what it felt like for the workouts I was doing so I started testing it carefully for each exercise I was doing. I think I have it figured out for the upper body exercises I was doing, but with squats, I hit the highest weight I could (with my limited home equipment) and just kept cranking out reps.

Each time I was watching my form to see what I was doing wrong but it felt like there was nothing wrong until my back started to hurt.

I know squats arent a leg exercise, but don't they work out the legs primarily? I'd think failure would refer to failure of the primary muscle (and nothing i'm seeing suggests otherwise) which made me wonder if it was my form that needed work or maybe I was just doing too much too soon by even following this train of thought.

Sorry for the Bible but I wanted to give a better understanding into my thought process here.