Backing up questions

I'm unsure about some backing up practices, specifically I am concerned about how optional it is. For all questions refer to this diagram:

XX enemy
AA ally
BB ally

Situation 1.
All units have the C standing order with no modifiers. AA is eliminated. Does BB have the option of backing up, or must the player have stated/indicated that fact? If AA is eliminated on its own turn BB may wish to declare the backup rule so that XX cannot charge on its turn, if AA is eliminated on XX's turn, then BB will probably not want to declare backingup as it would rather FR in its own turn. Am I correct in assuming the player has these options? In other words, there is no Backingup command that needs to be written on the card, it is purely player discretion.

Situation 2. Unit AA is spearmen is Holding on an objective, unit BB is also spearmen on Hold, does the player have to specifically say he is backing up unit AA?

I am assuming the option is open in both cases, as that seems to be the way it was played this past weekend. As handy as that was, it seems to confer way too much control on the part of the individual units. The fact that it can be a tough decision for the player should indicate that it is beyond the pervue of the unit commander, but the fact that it is a point of decision adds to the interest and depth of the game (i.e. fun). What I am saying is that I can see ample justification for going either way on this and I am unsure as to what the actual rule is, or more correctly, I am unsure what the rule will be in the next iteration.