Each feature in the model has a natural set of degrees of variance (DOV). For example, a planar surface has 3 DOV (1 translation and 2 rotations) and a simple hole has 4 DOV (2 translations and 2 rotations). The DOVs of each feature in the model must be constrained with respect to the features that precede it in the model. When there are not sufficient existing features in the model for constraining all of the DOVs of a feature, the feature should generally be set as a datum feature or be used to establish a datum from datum targets. For example, when you add the first feature to the model, there are no existing features to constrain the new feature to, so that feature should be set as a datum feature. Some feature types may not be set as a datum feature (see Feature Descriptions in Model Based Design > Geometric Dimensioning and Tolerancing Advisor > GD&T Advisor), but those features may be used to establish a datum from datum targets.

Generally, the first few features in a model must be set as datum features and are used to establish the predominant datum reference frame of the part. Thus, the features that you intend to be used to establish the predominant DRF should be ordered to be the first features in the model.