I haven’t done that plan, but am doing a plan I constructed from TFNA and think I have a grasp of the concepts.
I believe it’s telling you to do 10 reps of a weight that you could do 12 reps of. According to the UA folks, lifting until failure will stimulate muscle growth, which you theoretically want to avoid because you want to maximize your strength-to-weight ratio.
As someone approaching his late-30s, I am personally not trying to avoid muscle growth because I know it’s all downhill from here in terms of my body’s ability to build muscle. But that’s another discussion.