Rule 12.4 says that units entering a map are paid for by one CB marker per unit or through a die roll ( I assume that a 'no effect'roll of 4-6 is actually a failed roll and so you still have to pay with a CB?), but later the rule it says in the case of minefields and boxes the CB marker is not placed on the released unit.
You have two choices:
1- You pay a CB marker = the unit is automatically released.
2- You don't pay, you then have to roll a die for the unit's release. Whatever the result, you don,t have to pay a CB marker in this case.
So are the CB markers used for strategic reserve release spent and put in the used markers box or are they placed on the units?
They are spent and put in the used markers box, not placed on the units, as stated at the end of rule 12.4:
"Note that the Counterblow marker is not placed on
the released unit, but in the Used Markers box instead,
so has no other effect."