In Hole basic system the size of the hole is made constant, different fits is obtained by varying shaft size.
It is impossible to make anything to an exact size, therefore, it is essential to allow a definite tolerance or permissible variation on every specified dimension. Hence Tolerance is given to the part size to produce the parts, within the required permissible size error.
As per BIS system of Limits and fits, there are 18 grades of tolerances represented by number symbols from IT01, IT0, IT1 to 16.
The size of a component is given as 24 -0.1. In this -0.1 indicates Lower deviation which is -0.1 mm
In the assemblies obtained by using the system of Limits and fits, the damaged parts may be easily replaced without rejection the assembly.