Reference dimension

Reference dimensions are not intended to be used directly to define the geometry of an object.

Reference dimensions do not normally govern manufacturing operations (such as machining) in any way and, therefore, do not typically include a dimensional tolerance (though a tolerance may be provided if such information is deemed helpful).

Consequently, reference dimensions are also not subject to dimensional inspection under normal circumstances.

In Computer-Aided Design (CAD) it's commonly used to denote dimensions.

However, standard ASME Y14.5 has changed the way references are marked, and the abbreviation "REF" has been replaced with the use of parentheses () around the dimension.