[Seaside] Best practice: what actions in form callback: vs. in onChange: | onClick: | on...

[itsme213]

Some actions have fairly clear places e.g. if they need form values, or a 
rendering canvas or script to write on.

Other actions like model updates I am able to locate in many different 
places.

As good design, what kinds of actions should be done in <A> vs. <B> below?
e.g.
    html select
        ...
        callback: [:val |  <A> ]
        onChange: (html updater
            ...
            callback:[ <B> ])

Thanks - Sophie