In a specific type of fit, the difference between hole size and shaft size is called allowance. Allowance is used to explain the difference between clearance fit and interference fit. Positive allowance specifies the clearance fit whereas negative allowance in a fit specifies the interference or force fit.
The relationship existing between two parts, shaft and hole, which are to be assembled, with respect to the difference in their sizes before assembly is called a fit.
Allowance is used for proper mating and assembly
When the parts are assembled into sub-assembly units and sub assembly units are assembled into full assembly, the mating surfaces of different components are joined together for proper functional requirement. One of them may fit into the other in form of joint or fit. The fit may be with suitable degree of tightness and freedom for required relative movement between mating parts for specific functional requirements of the fit.