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A reference dimension is a dimension on an engineering drawing provided for information only. Reference dimensions are provided for a variety of reasons and are often an accumulation of other dimensions that are defined elsewhere. These dimensions may also be used for convenience to identify a single dimension that is specified elsewhere.
Reference dimensions are not intended to be used directly to define the geometry of an object. Reference dimensions do not normally govern manufacturing operations in any way and, therefore, do not typically include a dimensional tolerance. Consequently, reference dimensions are also not subject to dimensional inspection under normal circumstances.
Reference dimensions are commonly used in CAD software along with constraints that usually denote the opposite: mandatory dimensions to be precisely followed.