When it comes down to the nuts and bolts (and t-bars) of it, choosing holds is basically what we do. And boy is it important. It's important for all the obvious reasons - aesthetics, wall angle suitability, style consistency for flow - the list goes on. Every route setter understands this to a lesser or greater extent, but it's worthwhile to zero in on the relevant variables that are in play when we're choosing hold. To explain these variables, I want to introduce a concept from design called 'affordance'.
When designers talk about affordance, they are talking about a relationship. Specifically, the relationship between a person and an object.
An easy example from the everyday world is a mug. It 'affords' filling. That's simply to say, that's one of the ways it can be used by a person. Obviously though, this is just the function that it was designed for. It also 'affords' being hurled as a projectile, or even being used a door stopper.
In the climbing world then, when we're talking about holds and affordance, we're talking about which ways a hold can possibly be used. So what about this volume from HRT - placed on a vertical wall facing straight out - what does it 'afford'? In what ways can it be used by a climber?
You will probably go blue in the face before you exhaust the uses for which this hold has - but not so fast. Remember the definition of affordance - it is a relationship. Between the person interacting and the object.
So the uses for this volume are contingent upon what kind of climber we're talking about. Whether they're strong, tall, short, flexible, advanced, novice - whatever. For Adam Ondra, this hold (and just about any blob of plastic on a vertical wall) affords a huge amount of uses, but for somebody on the wall for the first time, it has a far more limited range of uses.
Now we can get to the important practical piece of advice here: Setters need to understand the abilities of the climbers for whom they are setting. In a commercial context, this will often mean understanding just what a climbers at each grade range are capable of - technically and physically. Or, even better, understand your specific customer base - their strengths and their weaknesses.. This is crucial when it comes time to selecting what holds we're going to put on the wall.
It's not difficult to understand why we need to understand the concept of affordance when we're avoiding making climbs too difficult for a specific grade range: Let's say we're setting a 5.10 and have a sequence requiring a deep lock off: if we choose a hold that only 'affords' locking off for a climber who is capable of a one arm pull up on a single pad edge, then the hold is inappropriate.
There's a trap looming here when we talk about affordance of holds: never get caught up thinking that because a hold can be used in a way you didn't intend (or missed entirely), that this indicates that there is something wrong with your route. It's only a problem if the different use, or skipping, makes the overall movement easier. To illustrate, consider this climb:
PURPLE CLIMB PICTURE
Chris Sharma, or frankly most climbers, could do this route using less than half of the holds? Is it a bad route? Should we take some of the holds off? Of course not. The route is designed to be a beginner route, that is an entry level of difficulty. The easiest way to ascend that wall is to utilise those holds in a jug position, climbing like a ladder. So, it's a good route. Just because you can dyno past the holds, or attempt to use them like slopey, undercling pinches, doesn't mean the holds need not be there. Things become more complicated when you start to debate a sequence on an advanced route. Say, wether dyno'ing past an intermediate crimp, is more or less difficult, but the principle remains the same. The intended sequence should be the easiest sequence (or equivalent to others, but not harder), and it should correspond to the difficulty for which the route was designed for.
When choosing holds, ask yourself, "Can this hold be used (in this orientation and on this angle) by a climber for whom this grade is their limit?" If not, it's a good bet that it's the wrong hold.
There are obvious caveats here: some climbers climbing at a specific grade range will be stronger in certain aspects than others, they will also have strengths in specific styles (overhanging, dynamic) whatever. It is not a law, but a guiding principle. The better you understand the affordance relationship as it pertains to holds and the climbers for whom you are setting, the better your routes will be.