ASME Y14.5 : Dimensioning and Tolerancing
Dimensional information, usually without a tolerance, that is used for reference purposes only. A reference dimension is a repeat of a dimension or is derived from other values shown on the drawing or on related drawings. It is considered auxiliary information and does not govern production or inspection operations. Where a basic dimension is repeated on a drawing, it is not identified as reference.
See Also:
Tolerance, Dimension, Drawing, Basic Dimension
Alternate Definitions:

PTC - A reference dimension is a calculated size used to show the intended design size of a part. A numerical value, enclosed in parentheses, provided for information only and not used in the fabrication of the part. Drawings made to older standards may use REF placed next to a reference dimension, instead of using parentheses.