Tags are essentially grouping of items. There's an infinite number of ways to group them, but for the sake of understanding each other, here's what has been the ongoing convention since 1.13 introduced them.
- Do not add tags to the minecraft namespace. Only add your things to existing tags only if you want your things to be used in recipes that might expect vanilla inputs. Example: only add something to the minecraft:wooden_buttons if your block is a redstone button, and is made of some kind of wood.
- Add your tag to the forge namespace only if it is designed to be grouped along with other tag files provided by other mods.
- Add your tag to some other mods' namespace only if it is designed to complement that mod's crafting logic / gameplay logic.