Basically if you used the creation kit for skyrim atleast once and have made a mod, you can fill in the survey. Ideally you will be able to list a mod you created using those modding tools as well. The size or quality of the mod isn't that important. As said in the OP it's not just for Skyrim, but any game that was released with some form of official modding tools (no matter how comprehensive) is valid. So most of the TES games and Fallout games qualify, as do the Total War games for example.