Some basic questions about share locations and cache

When designating the docker image and appdata folder locations, the help tooltip says the following:

"It is recommended to create this folder outside the array, e.g. on the Cache pool. For best performance SSD devices are preferred."

But the default is to create it on the array.  What is the difference, and is there a difference, between setting it to for example /mnt/somecachepooldevice/appdata/ vs the default /mnt/use/appdata/ and setting the share to cache only, set to that same cache pool drive under shares?  Are they equivalent? If not, which is preferred? 

In some form or another, this question has been asked and answered many times:

I see some ambiguity in terms of what works and what causes problems even though they should be one any the same.  The particular reason I am asking about this is that after moving my docker.img and /appdata/ folders to a new 2nd cache pool drive manually and setting the location to that drive manually in docker settings one docker does not seem to work: openvpn-as. Upon installing, its appdata folder is completely empty with the exception of two empty folders 'log' and 'etc'.  This was not moved from the initial appata location but was installed later for the first time. 

If I mount the docker.img file, the command

find /mnt/docker_temp/containers -name '*.json' | xargs grep -l /user/appdata 

yields two files: hostconfig.json and config.v2.json both belonging to the container openvpn-as.

So should I move everything back to /mnt/user/appdata from /mnt/cache2/appdata?  Or is this irrelevant and I should look elsewhere as to why there is nothing in the openvpn container?