How does it manifest itself, and how can one fix it/compensate for it?
Depends. Fixing it means correction which is not the same as compensation.
Parallax is the difference of orientation of an object viewed along two different lines of sight. Nearby objects have a larger parallax than distant ones when observed from different positions.
To correct: Move your scope until the objective lens is in the same position occupied by the viewing lens without changing the angle of your scope.
To Compensate: The image must be framed, and then the alignment of your scope is adjusted by tilting it upwards.
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