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Agree with everything @flynwill said except I believe E to be the line and valve for your wall sprayers. D is a bit of a mystery especially since it’s before the chlorinator. I wonder if it was an old line that was attached to a Polaris at some point in time and then converted to a standard return and that valve was installed in an effort to allow metering its flow. If you turn it off and lose flow from 1 return, that’s likely the case.As Jim said it is difficult to tell from your pictures what's going on. I will make a start but you will probably need to do some experimenting.
The pipe at the back of the pad that I've labeled "Goes to Waste?" look like it probably runs to waste water somewhere can you follow it? If that is true then valve A would be to select it and allow for vacuum to waste if you ever need to do that. It can also be used to pull down the water level a bit if needed after a rain. Does the pool have two skimmers? If so most likely valve B is used to balance the draw from the two. You can test this by moving the valve handle to two positions perpendicular to the pump in turn and then going to the skimmers and see which is still drawing water. There is also some chance this valve selects skimmer vs main drain, but he same test might verify that. Do not turn the handle such that it points away from the pump, that will dead-head the pump (shut off all flow -- there should be a stop on the valve body to prevent this but those are often missing or broken).
I am guessing that the line I labeled "Waterfall" feeds the waterfall feature I can see in your pictures. If that is the case turning valve C should cause either 100% water fall or no waterfall, as with the other valve do not turn the handle such that it points directly up. Since the waterfall water is not getting filtered I would adjust this for a minimal amount of water or only turn on the waterfall when there are people around to enjoy the effect.
I'm afraid D and E are a mystery to me without further information. I can only say that they appear to be returns of some sort. E might be for pressure driven cleaner but those usually have a booster pump rather than run directly from the pool pump.
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