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Circularity GD&T is used to vary the form of a Feature of Size, such as a pin or hole. Similar to as it may be found on a drawing, Circularity can be applied as a prequalifier on a Datum or as a refinement of another GD&T callout.
Creating Circularity Procedure:
1.In the GD&Ts dialog, select Circularity in the drop-down list and select [Add GD&T]. This will open up the GD&T dialog.
2.Underneath the Features list, select [Add].
3.Select the feature(s) whose form should be varied by this GD&T.
4.Click [OK][Close] in the Pick FeatureSelect dialog
At this point the GD&T dialog will pop back up. Notice that the features selected are now in the Features list in the top half of the dialog.
5.Enter the range in the text entry field.
6.Select [OK] to exit the dialog and the save the GD&T.
How this GD&T Varies the Feature:
A Circularity GD&T applied to a Feature of Size will vary the form of the feature. When deviating, the size of each cross section of the hole or pin will deviate independently of the other cross-sections while the center points of the cross-sections will remain co-linear. This differs from a 
Size GD&T where the sizes of each cross-section all change together as a group.
A sample feature with how a Circularity GD&T would commonly deviate it is shown below.
Note: While only a few deviated locations are shown below, the actual variations will use the entire specified zone.
A side view of a pin with a Circularity GD&T applied to it.
In accordance with ASME and ISO rules, Circularity can only be applied to a feature that is nominally round.
Note: Circularity has the same deviation routine as 
Cylindricity or 
Straightness with a linear zone on an axial feature.
Circularity as a Refinement:
It is common for Circularity to be a refinement of a location GD&T such as 
Position such as what is seen in the image below. If this is the case then 3DCS by default uses an RSS methodology to take the refinement into account without exceeding the limits of the location boundary. Settings for this can be changed depending on GD&T deviation method or Distribution input. See RSS Method for more information. An example callout for this is shown below.
An example of a callout where Circularity is a refinement of the Position GD&T.