A “Tag” is a subtype in D&D. For example, a Humanoid can also be a Shapeshifter, an Orc, a half-elf, and so on. Most subtypes can be from other main types or explicitly created to give more information about a creature. However, in the 5th edition, the Undead creature type isn’t added on to another creature type. If they’re reanimated, they simply become Undead.