On technical drawings, this tolerance is specified if it is to apply to surfaces that the toleranced surface is to lie between two planes.
These planes are at the specified distance from each other and are perpendicular to the reference.
If cylinder axes are toleranced, they must lie within an imaginary cylinder with the specified diameter.
The axis of the imaginary cylinder is perpendicular to the reference surface.
This glossary of plastic industry is provided by PLEXPERT Canada Inc.