Hello DougatHome,
Thanks for using Apple Support Communities! We see you have a question about the term "purgeable" and we'd like to help.
This page in the System Information User Guide has the information about used storage space:
See used and available storage space on your Mac
Scroll down to near the bottom and you'll see "The remaining space is free storage space. Free storage space is physically open space on a disk. Available storage space includes free storage space and purgeable disk caches (disk space might be both available and used at the same time)."
This is not something we can manage, as the macOS system will manage as space is needed.
Here's more info on storage space: How to free up storage space on your Mac
Take care!