It is a thing very close to tolerance but varies as allowance refers to tolerance in dimension that is intentionally provided in the metal during its manufacturing and designing. It is a pre planned deviation from the standard that is deliberately given to allow for adjustment if any untoward situation arises and clashes with the dimensions of the metal components or parts.
Furthermore, in the engineering discipline, much significance is given to tolerance. Metal parts are used a lot in manufacturing plants and assemblies. These parts need to fit each other easily otherwise their make is of no use. Moreover, this can only happen if they conform in size. To fit perfectly, metal parts or components require to have tolerance that lets them to expand or compress to some extent. The range of tolerance of a substance or metal depends upon the chemical and physical properties. The difference in diameters of the two parts is maintained normal for better fitting provided that two components need to fit into each other in a way that one will have to fit inside the hole of the other. In some cases, the gap is kept relatively small to let the force come into contention which is known as force fit.