A tolerance class is the shorthand of the ISO code system for tolerances on linear sizes (ISO 286-1): a designation like H7 for a hole or p6 for a shaft that, for any nominal diameter, corresponds to a pair of limit deviations tabulated in ISO 286-2:2010. Pair a hole class with a shaft class — H7/p6 — and the possible fit is fully determined: the interference range runs from shaft ei minus hole ES at the loose end (which can come out negative, meaning clearance, for the transition classes) to the shaft’s es at the tight end.
Interference-fit practice works hole-basis, holding the hole at H7 and selecting tightness by shaft class. The standard ladder: H7/k6 and H7/n6 are transition fits that locate precisely but can assemble with clearance at one end of tolerance, so they cannot be relied on to transmit load through friction alone; H7/p6 is the classic light press fit for parts transmitting modest torque that may be disassembled; H7/s6 the medium drive fit for permanent joints; H7/u6 the heavy force or shrink fit for maximum holding power. Because a batch of joints spans the whole tabulated range, designs take guaranteed capacity at the minimum interference and check stresses at the maximum.