Allowance

Allowance is a provision to allow for deviation between an actual dimension and an ideal dimension.

For example, suppose you need a hole for a 1/4" bolt. It is a common error to simply draw a 1/4" circle. Unfortunately you may receive your machined parts and find the bolt does not fit! The reason? No hole is exactly 1/4" - all manufacturing processes have variation and this is controlled by tolerance. If the tolerance is +/- .01" and the hole is at the low limit of .25 - .01 = .24, the bolt may not fit. To prevent this draw the circle of diameter .26. Then the low limit will be .26 - .01 = .25 which would clear a 1/4" bolt.

Similarly a 1/4" shaft that must fit inside a 1/4" hole should be designed undersized to be sure the shaft can be inserted.

The foregoing are just simple examples. You, the designer, are responsible for all allowances, to be sure your parts fit and work properly.