I am trying to work with labels, but having a few issues. Your guide says you can apply multiple labels to a file but if a file is labelled and I apply another label, there is no “stacking” at all. It simply removes the previous tag and the new one is applied. Furthermore, if I have the label column displayed, only one label is shown at a time. I have not applied stop on match. This is rather frustrating as the utility of labelling/tagging is minimal if you can only apply one label/tag at a time. I am using the latest opus 13.

You can stack labels, but it's not the default. See How to assign more than 1 label for a file? - #2 by Leo for how to change that.

But labels are mainly intended to change the colors of files, where stacking doesn't usually make sense. (Although it can, for example if one set of labels change the color while another changes the font, or similar things that don't overlap.)

Tags may be more what you're looking for, and allow multiple per file by default. You can set those via the Metadata panel.

Thank you. If labels are a file indicator, then aren't they a form of tag? I can see the meta data for labels in same way as I see tags or description.
I am a bit confused about how I get multiple labels to apply.

I'm not a programmer, just a mere mortal user...

Change it in Menu/Settings/FileTypes, you have to go through many dialogs

There's a specific Tags field that you can give a list of tag names (anything you type) to a file.