What we mean is the assets that come with a purchase of an RPG Maker engine. (Not including DLC's if you bought the engine in a bundle. Just the engine and the assets it comes with alone)
Golly, you can use the ones in the dlc/baseassets folder, and anything that generates with a fresh project. The point is use content that everyone has. If you went to steam and purchased a copy of MV (or any other rpgmaker engine) by itself (unbundled), the files included with that purchase are usable.