For questions about the nature of magic in a game. Use [spells] for questions on specific spells.
For questions about the role and nature of magic in a game or setting. This covers more than just spells and spellcasting, including the nature of magic, and general rules regulating magic.
The magic tag is about magic as general a concept, system, quality, or so on in gameplay, mechanics, or game lore.
Note that the magic tag isn't aiming to be a catch-all for every single question about magical things, so in general if you're asking about something more specific that is also incidentally magical (such as spells or magic-items), you don't need to use this tag, unless you are also asking specifically about magic as a concept/system/quality/etc.