Exactly what mysthalin said.
Purchase your infantry and deplete the supply pool (by purchasing another 10 points worth of units on top of your pool value), then purchase units for the armour pool? =)
And if i have to resupply my battalion, because i lost alot of infantry but no sherman, will the pp cost remain on the armour category and the infantry will automatically be again reserve pool? Because deleting Shermans with veterancy just to have them not cost PP so that the order of the units purchased makes infantry cost reserver pool points?
Why not make it selectable,instead of letting the system decide where to use the reserver pool, we can leftclick/rightlick on a category to spent/withdraw our reserve points from there, this way the reserve pool points always will be used where we want them to be used.