Scrub, from_scratch, zero Plugin. Rewriting all but specific metadata.

[LeftAtThePool]

Hi, Im quite new to beets;
Since I am looking to replace lower quality files in my collection in the future, I would like to keep a record of those files before consolidating my collection. Currently I am using the comment tag to store information about the source of those files and would like to keep the comments upon import.
Also I am looking to use only minimal metadata (Apple Music+what is needed for Picard); Is this what the zero plugin is for? Is there a difference between deleting metadata with scrub or the from_scratch plugins and nullifying it? Is the beets tracked metadata the same as Picard's. Is there a list of it somewhere?

Thanks and Cheers!