I were to manually delete these files, to Everything this tester instance does not exist?
An Instance is only a "logical" concept, if you will.
An Instance (by default) names its set of "data" files; *.ini, *.db... with the specified Instance name.
So if the Instance is not running, & you delete said Instance related data files, the "instance" does not exist (at that point).
(Fire up Everything with that Instance name again, & well, it lives!)
For example when I create a instance I have to keep specifying the DB location
For me, I use the ZIP's, so by default the DB files are located in the directory (I chose) to use for any particular Instance.
(The installer defaults to %appdata%, or where ever, & I'm not really familiar with that.)
I create various directories:
/test1/, /test2/, /test3/ & copy (an) Everything.exe into each.
Then I'll typically thrown in a GO.bat to open each particular instance.
start "" Everything.exe -instance test1, in /test1/
start "" Everything.exe -instance test2, in /test2/
start "" Everything.exe -instance test3 -read-only, in /test3/
With that, Everything (everything) is pretty much self contained to a particular directory.
Likewise makes it easy to test various versions of Everything simply by using a different version of the Everything.exe executable.
If I want to make a backup, (exit that particular Everything Instance) & make a copy of a directory; test1 -> "test1 (copy)".
(Firefox used to work in much the same way, with Profiles.
But they, of course, bastardized it horribly, so one can no longer be confident as to what might or might not happen.
[Actually, it kind of still works - if you bypass their "Profile Manager" itself & just do things "manually" instead.
I have a "go.bat" for that too.])