Engine / instrument used in creation of the gallery - id est: "Photoshop", "Maya", "Koikatsu", or "Skyrim" - should NEVER be used in tagging regardless of anything else.
@Pseudo-Nim I'm not familiar with the detailed tagging rules for this case (and I didn't vote on this gallery's tags or anything) but for the record, I think when people create a tag like "Koikatsu" they are referring to character models and assets. So for example, these are Koikatsu models. Skyrim also has its own sets of clothing models, environmental models, etc. which are distinctive. Other examples might be Daz 3D/Poser, and Custom [Order] Maid 3D 2. It's also possible for the engine and model to differ, for example, Custom Shoujo 3D models being imported into MikuMikuDance (like what Boko877 does for his renders). I can agree that nobody should ever tag Photoshop or Maya, though, or rather, to do so would be nonsensical.