Apply action to multiple/all fields?

[ohrenkino]

@ngervae

an action should apply to multiple fields

I find this a little theoretical - a real example would be helpful.
Here is a thread for the WIndows version that deals with the same action for several fields:

Specifying Multiple Fields for an Action

Back 2011, somebody ask if it's possible to specify multiple tags for an action and somebody replied it's not possible at the time. Is this possible now in 2017? If yes, how can I do it? I tried fields,field2 as an example and it wouldn't work. It's possible I'm using the the wrong delimeter (looked for examples on the mp3tag website and can't find it). It's also possible that this feature is still not available. Thanks

Please note that the Windows action "Guess value" is called "Import tag fields" on the Mac.