"Tactical" and "local" reserve...

[Currahee]

A unit in tactical reserve will move, during the combat phase, to an adjacent, friendly hex, to support a cooperative unit (same formation, or otherwise fully cooperative...)....It is a good way to shore up a defensive line, if you have enough units.....for example, you have three hexes to cover, and four units; if you had six units, you'd dig in two on each hex, but you only have four; so you dig in three units, one on each hex, and the fourth, set behind them and adjacent to all of them, and set on tactical reserve; whichever hex is attacked, it will move onto that hex during combat, and help hold the ground. But it won't defend entrenched, so if you know which hex will be attacked, or there is one critical hex you absolutely must defend, it's better to go ahead and entrench on that hex, rather than use tactical reserve.

Local reserve is different; the unit won't participate in combat, but will move toward the combat. I don't find it too useful, but I think it helps the PO play a better game...