Numbers In Use vs. Number Inventory

We went live on RC late-May - Office and Contact Center. We ported over all of our DID blocks at the time to make sure we didn't lose something in the transition. We are preparing to delete many of those now-unneeded phone numbers. All the numbers are in the Numbers in Use and you can call the numbers and be dropped into our main IVR. What is the difference between having the numbers "In Use" and in "Inventory"? Is there a cost difference? Thank you.