Editing actions from NLA editor. What's saved?

When I insert action in NLA editor and it becomes a strip, and then enter strip edit mode an it turns green,
will the changes made for action be saved only in that instance that is already in NLA editor, or it affects the
original action that stays in library too?

Any changes to the action strip will affect any item that uses that same action. You will need to create a single-user instance of the action to make changes affect only that user.

Alternatively, you could keep the action for all users, but create another overlapping action. Then choose to have the new action blend by adding, multiplying, subtracting or replacing the underlying action.

Hope this helps.

SB

Ok, yes, so it actually changes the original action except if I make a new instance before I start editing the strip/action.