Using The RIR Scale

RIR stands for Reps In Reserve. 

It is used in 2 ways.

1) I can ask you to record an RIR rating at the end of an exercise and it lets me know how many more reps of an exercise you could have done for your sets. 

2) I list an RIR as how challenging I want the exercise to be for you. 
This means I won’t necessarily list an exact weight for you to load to the barbell, you will get to choose that and can base it off your past training history and what is appropriate for your training that day.

This is commonly used if you've had a break from training, or if you've been a little bit unwell, or if there are other variables that warrant its usage and in either case it gives you the flexibility to determine a safe and appropriate weight for an exercise that day.

Here is how in works:

Me Prescribing RIR Exercise.

You may see me write in guidance to do an exercise that would look like this:

Squat: RIR 2 x 5 x 3 

which means select a working that you can do (that leaves 2 reps in reserve) for 5 reps and 3 sets 

OR it could be written 

3 x 5 @ RIR 1-2 

which would mean 3 sets of 5 reps leaving 1-2 reps in reserve that you could have gotten done one each set.

You Giving Feedback on an Exercise.

you may be doing the squat and write down numbers such as this:

100 x 5 5 5 RIR 3

which tells me you got 100kg for 5 reps and 3 sets and could have done 3 more reps on your sets.