You've answered your own question, with a good choice of words!
You can put projects into categories. Log in as an admin, and go to Admin -> Projects -> Project categories, and add some. Then go to each project's admin section and select a category for them.
I'm afraid it is a single mapping, you can't put a project in more than one category, but it does the basics well. (I'd like to see project labels myself, so I can put several labels on projects and search by them)
The category does not do much itself, there's no effect on config. But you can run JQL like "Category = X" to find issues across a group of projects, which is sometimes more useful than you expect.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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