The Auto-Assign Bone Joints Window allows you to quickly assign vertices and points to bone joints. The two factors that determine which bone joint a vertex or point is assigned to are the orientation of the joint and the distance to the joint.
The Only assign to selected joints checkbox indicates if you want to assign to any bone in the model, or limit the assignments to joints that are selected. If no bone joints are selected, this checkbox is disabled.
The auto-assign feature will assign a primary bone joint influence and up to two additional influences (a child and parent of the primary joint) for a total of up to three bone joints. The Single/Multiple slider allows you to increase or decrease the range of child and parent bone joints. If the slider is completely on the Single side, a vertex can only be assigned to one joint. As the slider moves closer to the Multiple side, child and parent joints of the primary joint are more likely to be assigned.
Press Ok to auto-assign bone joints or press Cancel to leave bone joint assignments as they are.