The simple example for how the draft slotting works. Imagine having 3 picks as slots A, B and C. The commish's office assigns slots:
A: $3,000,000
B: $1,000,000
C: $500,000
This means your total bonus pool is $4,500,000. This is bonus money for ALL of your picks. And you only get the $4.5M by signing the 3 guys. Now suppose you DO sign the 3 guys for the following bonuses
A: $2,900,000
B: $1,100,000
C: $250,000
You signed them for a total of $4,250,000. This means you have $250,000 to spend on bonuses outside of the slotted rounds (first 10 rounds). So you could try to lure a Middlebrooks or Westmoreland from a harder commitment with the $250,000 for instance.
Now what if you cannot sign the B slot guy. Then your max pool drops to $3,500,000 in this case. That money is gone. The system forces you to take a few easy signs to keep you in the game on the harder guys.