Repetition maximum for exercises

Not necessarily related to rehab, basically just wanan know/confirm regarding this. We usually decide on the weight or resistance based on the RM is it not? Usually 10 reps? If one is able to perform 10 reps then that weight is the one to start off with? Example biceps curl 5kg 10 reps. We then give about 2-3 sets right? What if during the 2nd set one is not able to complete the 10 reps? Is the weight then considered too heavy and should use a lighter weight? Or as long as you're able to complete 1 set of 10 reps then that is the ideal weight we should target?
Hope you guys understand, don't really know how to explain this. Am quite confused myself.

A RM can be determined from a 1RM, 3RM, 5RM, 10RM....whatever (not necessarily just 10RM).
If you are doing 10 repetitions of an exercise, you should be able to get through all your repetitions in a set but it should be hard work. i.e. if you blow through 10 reps with no difficulty you could increase the weight; if you can't finish your 10 reps you should decrease the weight. It should also be an appropriate work load that after a rest between sets you should be able to get through the all 10 reps again.

Ok. So even after adequate rest, and still unable to complete the 2nd set of 10 reps, means weight should be reduced eh. Hmm....understood. Thanks mate.

Adequate rest times should indeed be taken into consideration.

After a 10RM set, it is likely that available ATP and PCr sources are utilized. Normally at least 2 minutes rest is required for recovery between efforts. Additionally nutrition will impact significantly on the ability to perform second sets. Without muscle glycogen stores at high levels each subsequent set is likely to be more difficult. So stick with the 2nd set at 9 reps. Add weight when both sets can be completed for 12 reps. Objective markers are necessary,