The 50” at 100 yards is the one I don’t get.
Everyone here has answered correct.
But, this can be a confusing subject.
It's already been stated in different way~s,,,,
I'll add, 1/8 click = means 8 clicks for ONE full MOA.
So if the scope is advertised as having 50" , then that is 400 Total Clicks from one side of the turret ( Elevation or Windage ) being run to the stop ( and NOT with a Pipe Wrench).
Why from one side ( one at a time, by the way)?
Because of the curvature of the scope tube.
One axis has to be in or close to the middle, otherwise you'll not get full travel from the erector tube.
I know Grim knows the following, but I'm split off from his description of center.
If the erector tube is in the middle ( or centered ) you should be able to obtain 200 clicks to either side, or 400 total from one side.
25" Up / down & 25" side to side ( 200 clicks ) will or should put the scope AT Mechanical Center.
So, what I'm sharing is, Mechanical Center is NOT necessarily Optical Center.
It should be but if glass have moved internally or other issues are at hand than an Optical & Mechanical center may not be the same.
Ideally, having a scope at or starting from Optical / Mechanical is a very good thing because you can judge the firearms condition ( meaning if things are straight / shooting straight ) while ruling out the optic as a possible problem, or not.
Safe Shooting