Darn it, Simmox beat me to it...
There are two columns of numbers on the left hand side of the screen when purchasing units. The first (left) column denotes the points value of the units and the second (right) column denotes the rarity value of the units. You will see that each unit has two numbers nect to it, the first (left) is the points value and the second (right) is the rarity value.
The columns on the left hand side of the screen are then split into three rows. The first (top) is the amount of points you are allowed to spend in the scenario. The second (middle) is the amount of points you have spent already on units. And the third (bottom) is the amount of points remaining to be spent. This applies to both points value and rarity.
So if you start with 1000 points for units and 1000 points of rarity you would see:
1000 / 1000
0 / 0
1000 / 1000
If you then purchased a Tiger for (at a guess) 500 points and 750 rarity points you would see.
1000 / 1000
500 / 750
500 / 250
As long as the BOTTOM numbers are both green, you may proceed with that currently chosen force. If one, or both, are red, then you have overspent and need to reduce the cost of your force.
make sense?