Usually these "Kao's" are something like a monogram made of the Artist real name or "Artist Name". But the end design is entirely up to the artist.
Basically a "Kao" is the same as a Signature or "Mei" on a pen. There are no set rules when to use one or the other. For a very formal importnat work, the artist will use all three: the guild name, the "Mei" and the "Kao". If the item is less importnat, the artist may only use the Guilds name and his "Mei" or he may only use the "Kao".
Directly on the tea dogu is usually written only "Kao" of certain person (like budhist monk or Iemoto) to show that he/she made the item or that he/she did approve that the item is a fair quality. On the outside of the wooden box of important item is ussually writen the name of the artist who made it and on the inner side of the lid of box is writen poetic name of the item (if it has some) and "kao" of Iemoto who approved the quality of the thing. Writing on the box is called Hakogaki.