It'd be nice to mark a custom field as permanent so that you can keep a comment about what an IP address used to be for historical purposes. This would not get overwritten unless you want it to even if an IP is marked as available. Everyone has some devices that are pointed to old IP addresses that have been re-provisioned. Knowing what used to be on there is one of the things that I used a lot in the IP spreadsheet days that would be nice to carry over.