Reserves - From a purely gameplay perspective I have found the traditional concept of reserves does not work particularly well in MTW2... It is next to impossible to withdraw depleted units from the line while replacing them with fresh ones and trying to plug a heavily depleted and wavering front line with piecemeal insertions of fresh units is a receipe for a mass route...

I have found two effective uses for reserves that are basically similar in concept.

The first is more defensive and is the second line, often of more professional (read higher morale/elite) units. Once the first line starts to waver in places you can attempt to withdraw it behind the second line. The second line bolstered by your general should hold and once the withdrawing units are clear they can be stopped (and the general can attempt to rally any routing units). Once the units are reformed, the more intact ones can be returned to the line on the flanks of the existing units.

The second is the counter-attack (or second wave if you are attacking). In this case your second line will ideally consist of sword or axe units and once the line battle has matured you can charge in the units of the second line. either in mass or in targeted locations to either reinforce a suffering unit or to take advantage of gaps appearing in the enemy line.

In effect once you have your second line in place it can be used in either of the two ways above as required, it can also be called on to assist in defending the flanks.

I guess overall what i am saying is that the is little point in holding one or two units at the back and calling them reserves. What you can achieve with them is extremely limited. This second line should be at least three units of quality infantry that ideally will do little fighting if your other tactical and strategic gambits succeed. If you played MTW think Triari, but more dynamic...

This deals only with the line of battle, in MTW2...