A tolerance class like H7 or g6 packs two ideas into a few characters. The letter sets the fundamental deviation — where the tolerance band sits relative to the nominal size (the zero line). The number is the IT grade — how wide the band is. Put a hole class and a shaft class together, at a given nominal size, and you have a fit: the range of clearance or interference you'll actually get.
At Ø50, H7 gives a hole of 50.000–50.025 mm (deviation 0 to +25 µm). g6 gives a shaft of 49.975–49.991 mm (−25 to −9 µm). The loosest pairing leaves 50 µm of clearance, the tightest 9 µm — a classic close-running fit for a rotating shaft.
Swap the shaft to p6 and the zones overlap: instead of clearance you get 1–42 µm of interference — a press fit that has to be forced together.
Clearance: the hole is always bigger — parts slide. Interference: the shaft is always bigger — parts must be pressed or shrink-fit. Transition: depending on where each part lands in its band, you might get a slight gap or a slight squeeze — used for accurate location.
Hole-basis keeps the hole at H (lower deviation zero) and varies the shaft letter — the usual choice, since holes are harder to machine to size. Shaft-basis keeps the shaft at h and varies the hole. Both reach the same fits; hole-basis needs fewer reamers and gauges.
Both the IT grade width and the fundamental deviation grow with nominal size — a 10 µm band on a 5 mm part would be impossible on a 400 mm one. ISO 286 scales them using the standard tolerance unit, which rises with the cube root of size.
Start from the function: free-running (H8/d9, H9/d9), close-running (H7/g6), location/transition (H7/k6, H7/n6), press/interference (H7/p6, H7/s6), drive/shrink (H7/u6). Then confirm against your load, material and assembly method — this tool gives the numbers, not the engineering judgement.