In precision mechanics, fit refers to the degree of 'looseness' with which a shaft is inserted into a bored hole.
This coupling is related to the tolerance or allowance of both parts' dimensions.
The shaft and the orifice must be of a similar diameter, otherwise there will not be a correct adjustment.
With this in mind, measurements have been internationally standardised according to ISO regulation to ensure the interchangeability of items and their mass production.
The lower the value the higher the machining costs, as a greater precision is required.