Karl Voit has a very nice post on Tags versus Categories in Org-mode. I’d completely forgotten about categories, even though I’ve written about them before, so I was glad to get a refresher. Categories, for those who don’t know, are the names in the left of the agenda listing. I always thought of them as just the name of the file the item was from but that’s just the default. It turns out that that name is really the category and you can set it to anything you want.
The question that Voit considers is when to use categories versus when to use tags. There is, of course, no hard and fast rule but Voit explains what he does and why he does it. Generally, he just uses the default category but sometimes he may have several files related to a single “thing” and he uses a single category for the files so that all the items have the same name in the agenda. That’s more important than you might think because you can filter the agenda on a category so you want related items to have the same name.
Whereas each item can have only one category, it can have several tags. Voit recommends using a limited number of tags but emphasizes that that’s just what works best for him. I tend to use tags as a list of keywords to help me find an entry. Thus I might tag a journal entry containing the magic spell to compile Emacs with emacs and compile. I try not to go crazy and use more than, say, five tags per entry but I haven’t found that to be limiting.
The moral here is that there are many systems for tagging and using categories and you should choose one that best fits your workflow. Be sure to take a look at Voit’s post. It covers all the details and is very informative.