Each feature in the model has a natural set of degrees of freedom (DOF). For example, a planar surface has three DOF (1 translational and 2 rotational) and a simple hole has four DOF (2 translational and 2 rotational). The DOF of each feature in the model must be relative to the features that precede it in the model. When there are not sufficient existing features in the model to constrain all DOF of a feature, the feature should be set as a datum feature. Or it can 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 but those features can be used to establish a datum from datum targets.
Generally, the first features in a model are set as datum features and used to establish the predominant datum reference frame (predominant DRF) of the part. Therefore, the features used to establish the predominant DRF should be ordered to be the first features in the model.