In order to properly constrain the features of a part, you must establish a predominant datum reference frame that is sufficient for constraining all of the degrees of freedom (DOF) of the part. Parts generally require a predominant datum reference frame that constrains 6 DOF. An exception is when a part is symmetrical, such as a simple sphere or a simple cylinder, in these cases there may be degrees of freedom left unconstrained in the predominant datum reference frame because they are not required to define the part.

When setting the datum features for the predominant datum reference frame, you should generally select the part features that orient and/or locate the part in the assembly. Those features should be the first features in your model.