Allowance is the intentional difference between mating sizes; tolerance is the permitted variation.
Allowance is a design decision — the minimum clearance or interference you intend between two parts. Tolerance is a manufacturing limit — how much either part may deviate from its nominal size.
A fit has one allowance and two tolerances (one on the hole, one on the shaft). Confusing the terms makes a drawing ambiguous: "allowance ±0.05" is meaningless, because allowance is a single signed value, not a range.
Using allowance and tolerance interchangeably. One is the intended gap, the other is the permitted error.
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