"AI-generated assets are only allowed if you own the training data. (e.g. Photoshop's content-aware fill, training an entirely separate instance of AI on exclusively your work, using ML to remix art with permission from the artist, etc.) Systems in which the training data is not owned by the dev (ChatGPT, Midjourney, Stable Diffusion, and similar) are not allowed at this time" [From NanoReno page]
An asset - property is anything within the game itself as clarified already by sake. This includes, both physical assets (VA/Art/Code) as well as IP assets (Ideas, style etc).
This is a bit of a quagmire, and I think really the rules should be updated to just ban use of prompt based AI outright. As the exception is a bit of a grey area. Also when you license the use of an AI, you are potentially also licensing the training data, which is a legal argument I really don't want to talk in detail about as its a waste of time pivoted off of legalese and semantics.