From the AS help:
Well, I've discovered that you can use manual binding and automatic binding at the same time and to the points that you want.Flexible binding (the default for new bone layers) uses a method of bone binding where every bone has some influence over every point in a vector layer. The farther away a point is from a bone, the less influence that bone has over the point. However, you can still experience some rubbery movement where a moving arm can cause a toe to twitch a little bit.
With region binding, on the other hand, every bone has a cutoff radius, outside of which it will not affect the movement of vector points. This can be used for cleaner movement, but requires a little extra setup.
Bone Strength
...
Manually binding points to bones is a process that exists mostly for compatibility with older versions of Anime Studio. Automatic binding, combined with bone regions of influence is the easiest way to work with bones in Anime Studio. However, there may be some times where you want more control over exactly which bones control which points.
...
Although this method does work for binding points to bones, we don't really recommend it unless you have a special need to bind specific points to specific bones
...
Manual point binding can also be very labor-intensive, especially when your artwork has lots of control points. Finally, automatic bone binding can be used to warp image layers, while manual binding can't - another reason to stick with automatic bone binding (using either flexible or region binding mode).
Steps:
FOR MANUAL BINDING
1) Select the bone.
2) Select Bind Points (I)
3) With the mouse (manually, lasso mode or what ever) select the points what you want to be binded.
4) Press spacebar or the Bind Points button.
http://amanoalzada.iespana.es/bindpoints.html
Now if you want ALSO that this bone take control of other points in a "flexible way" you can do this:
1) Select the bone.
2) Select Bind Points (I)
3) With the mouse (manually, lasso mode or what ever) select the points what you want to be flexi-binded.
4) Go to Menu Bone->Flexi-Bind Points.
http://amanoalzada.iespana.es/flexibindpoints.html
You can notice that there are points in the influence region that still being not moved by any bone. The points that are under right bone and vertical one have influence of both so they don't move as a rigid solid. like the points on the right and the left (teh ones from the brown shapes)
I think this feature opens a lot of posibilities to make animation more rigid in some areas and flexible in others.
NOTE: Region Binding is selected instead of Flexible Binding in Bone tab of the bone layer options window in this example.
Hope this tip helps.
Genete