All model instances are treated as rigid bodies. As a rigid body, a model instance initially has 3 degrees of freedom (DOF): the x position, the y position, and the rotation. Therefore, even if the master model was fully constrained (DOF equals 0), when you first load the model instance into a file, the DOF will always equal 3.
You can constrain an instance in the same manner as any other geometry. Any geometric element of an instance can be constrained to other geometric objects, including elements of other instances. See Placing constraints for information about constraining one geometric object to another.
Individual objects in model instances are not directly available for constraining. That is, they cannot be selected and manipulated from the sketch or from the parameter table. To constrain individual objects within an instance to other instance objects or to assembly objects, use Connections.
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