What tolerance means
Tolerance is the acceptable range of variation from an exact specified dimension — a component specified at 50mm with a tolerance of ±1mm is considered correctly made anywhere between 49mm and 51mm. Every fabrication process has some tolerance; the question is how tight it needs to be.
Why tighter isn't always better
Extremely tight tolerances require more precise (and expensive) equipment and more inspection time — specifying unnecessarily tight tolerance for a component where a millimeter of variation is invisible to the eye just adds cost without a real benefit.
Where tolerance matters more
Fittings that need to mate precisely with other components — glass clamp fittings, modular panel systems, anything with moving parts — genuinely benefit from tighter tolerance specification, since a poor fit here is functionally, not just cosmetically, a problem.
Setting realistic expectations
For most architectural railing and fabrication work, small variations within normal fabrication tolerance are completely standard and not a quality defect — if precision is unusually critical for your specific application, raise it explicitly when ordering rather than assuming it by default.