system prefernces-storage shows 100 GB available, but disk utility shows MacIntosh HD-Data only has 20 GB free. Why the difference? (Big Sur)

Terminal shows the same info as Disk utility for the "Container disk1 APFS container". As I am trying to piece together the terminology, this is what I currently believe:

Disk utility for Macintosh HD-Data:

uses the word "FREE" to mean disk space that is totally unused, ie, nothing in that space now.

uses the word "USED" to mean disk space that is currently filled with user data

has a catch-all term of "OTHER VOLUMES" which includes the Volumes of the system (Macintosh HD) data, reboot data,

recovery data, and whatever VM data is.

so, in my case, I have basically FREE of 22 GB, USED of 211, and OTHER VOLUMES of 18 to match total HD size of 251

but, Disk utility for Macintosh HD-Data also shows Available 116 GB (94 GB purgeable).

So my belief is that Available is PURGEABLE plus FREE.

So my questions are: What does purgeable mean? Is this merely my cleaning up trash, old movies/pictures/etc? How does Apple/Mac determine what is purgeable? It seems to me that if Apple can come up with a precise number (the 94 GB), that there ought to be a way to see what is purgeable. Using the System Preferences-->Storage-->Manage Storage is probably how it works, but I am not sure.