There were debates on cgi before and I think its use is still quite fuzzy.
Technically, all animated content you see on this site are more or less Computer Generated. I feel like people tend to use the tag on realistic-looking graphics as there’s a history of the technology’s pursuit for realism, as seen in the film industry.
But then due to the definition it is also not wrong to use it on toon-shaded 3D animation or even Live2D. There’s in fact quite an overlap between mmd and cgi on this site - most if not all mmd videos aren’t realistic at all.
No matter which approach you take, the cgi tag isn’t curated well imo. Hundreds of 3D animation topics does not have the cgi tag.