Protection / Slot Rank:
Make no mistake that these are passive boosts. Unlocking these on the upgrade menu will only enable you to purchase them on the actual missions. Basically, you still have to pay for slot upgrades on the mission themselves. Also, the last two branches of this upgrade tree mainly reduce their purchase costs.
Protection 50%
Halves the damage dealt by the Zombies to your units standing on slots purchased with slot protection.
Slot Rank 2
Increasing a Slot Rank will render a unit standing on it better reach, better fire-rate, as well as better firepower.
Slot Rank 3
Similar effect to Slot Rank 2, only with superior boosts to the unit standing on it.
Cost $1000
Reducing the cost of an orange slot rank from $1500 to $1000
Cost $2000
Reducing the cost of a red slot rank from $3000 to $2000