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By upper plane, you mean -A- is the surface that the hole you are inspecting is drilled in to? If so, A | B controls everything except translation up/down as drawn.
If you mean -A- is the surface -B- is drilled in, then -C- is restraining rotation around A and B, since B won't help there and all degrees of freedom are constrained.
As far as what I'd choose, I generally look at how it was made. In a vise side milling the two planes making -C- and drilling the hole you are inspecting? Hole is relatively shallow? Circle.
As described above, but hole is LONG (compared to the diameter, 5 times the Ø or worse I consider long), cylinder.
-C- made on a different operation than the hole in question and it is deeper than 1/4 inch? cylinder. Shorter than 1/4 inch, circle.
-C- made on two different operations so the planes might not even be parallel? Cylinder no matter how deep.
Hole is crossing -B- so it has interupted cuts and it is a high speed steel drill? Cylinder.
Hole is crossing another hole (not as you pictured, but off-center so the drill is hitting on like 100° to 300° of it's diameter? Cylinder.
Hole is threaded? Circle.
Hole is threaded and engineer thought it was cute to put a projected tolerance zone? Cylinder with a LOT of profanity.
Made by helically boring with an end mill and then reaming with two reamers (or three) stepping into the size? Circle.
I don't know how it was made? Cylinder.
Then, as Sid said, open tolerance compared to the depth of the hole? Circle, so long as that interupted cut thing didn't rear its head or it is really deep.
Tight tolerance, cylinder.
Without C, both red and green holes would be ok...
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Measuring as a circle should be enough (IMO)
But if I measure as a circle (thus projected in I vector defined by datum A and C) what is the point of using the C datum in the callout all together? Wouldn't it be the same to write A/B only? (I am not questioning your suggestion btw, just wanna learn how this works ; Learning from this example/post is more important that the actual problem I am facing).
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