A Dynamics Group is a way of ensuring certain dynamics objects only get affected by certain other dynamics objects. In each dynamics options panel, there is a Group drop down menu which has all the dynamics groups created so far listed, and also <default> and <new group>. If <default> is selected, then the current item is affected by all other dynamic items in the scene, and affects all other dynamic items. If <new group> is selected the user is asked to name the new dynamics group created, and the item is placed in that group. Items in a group are affected only by, and affect only items in the same group, or items in the <default> group. You can change an item's dynamics group in a number of ways, including in collision with a Collision item with its Change toggle set.