Hidden units is an option, and the general intention is that where hidden units can be deployed is based on the scenario when the option is chosen. Along with how many (if any), what type of units, etc. So the hidden unit rules do not specify a limitation because it's scenario specific.
Usually you need something to say you can hide units, and that should also include the above. For example, Tactical Experts (Hidden Units) says "on their half of the map, at least 12" away from any enemy units." The Reconnaisance scenario option Deadly Hide and Seek allows one-third of their total force as hidden units "in and about the central urban area on the map."
So hidden units does not have an overall hidden units must be used only in X, the ability that allows hidden units should specifiy where they are allowed.