Not All DOF of the Design Model Are Constrained
In order to properly constrain the features of a part or assembly model, you must establish a predominant datum reference frame that is sufficient for constraining all of the degrees of freedom (DOF) of the design model. Design models 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 features that orient and/or locate the design model in the assembly. Those features should be the first features in your model.