Reference dimension
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 (e.g. on the drawing or other related documentation). These dimensions may also be used for convenience to identify a single dimension that is specified elsewhere (e.g. on a different drawing sheet).

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.

Reference dimensions are commonly used in CAD software along with constraints that usually denote the opposite: mandatory dimensions to be precisely followed.

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

REF 
Prior to use of modern CAD software, reference dimensions were traditionally indicated on a drawing by the abbreviation "REF" written adjacent to the dimension (typically to the right or underneath the dimension). 

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. As an example, a distance of 1500 millimeters might be denoted by  instead of . 

This implementation has followed in modern CAD software that makes use of parentheses as the default denotation method whenever reference dimensions are "automatically" created by the software. The method for identifying a reference dimension (or reference data) on drawings is to enclose the dimension (or data) within parentheses.

See also 
Engineering drawing abbreviations and symbols
Geometric dimensioning and tolerancing
ASME Y14.5

References

External links 

 Y14.5 Dimensioning and Tolerancing, 2018, ASME

Technical drawing