About Mechanism Design Rigid Bodies
A rigid body is a group of parts that are rigidly controlled, with no degrees of freedom within the group. The connection sets used to place a component determine which parts belong to a rigid body.
During the assembly process, connection sets are used to define the relationship of the component you are adding to your assembly. There are two types of connection sets: user-defined connection sets, such as Coincident and Normal, and predefined connection sets.
Ground bodies in a mechanism do not move. Several parts or subassemblies may be included in the ground body.
* 
Features or datum planes belonging to rigid bodies with external references to other rigid bodies may move with the reference rigid body when dragged. The position of such features or datum planes is corrected after the model is regenerated.
A large number of rigid bodies, can lead to performance problems. It is recommend to keep the number of rigid bodies to a few dozen. Exceeding 100 will considerably slow performance, over 150 and performance can slow to a crawl. Performance depends on the number of rigid bodies, the complexity of the mechanism, and the hardware being used.