[Mobhack]

The scenario designer can set reinforcements in a scenario, for either player. That applies to stand-alone scenarios, or a user campaign, as those are a linked series of scenarios.

In a random battle, including long campaigns, the AI will reserve some troops to enter later as reinforcements. These AI troops are handy e.g. for catching human scout cars beating up on it's gun line. If you want that effect - place them in the rear and just don't use them for a while.