With this property you can link all sorts of different otherwise unrelated elements. You could for example use this as part of a bigger change you need to do to your code base, marking all the bits you need to modify.
Adding a tag is as simple as just using it:
For Visual Studio to find places where the tag is used, you need to do a build with the "Build cross reference data" option checked:
And here is where you search for tags:
And here is the result:
This ought to work for code to. The editor knows about the tags and has intellisense for them:
But the "Find tag references" tool doesn't pick them up. I wonder if that is a bug or not.
And in my opinion you should try to not go overboard tagging away all day long.