Nope, I just have one field too, same as you.
I'm guessing this table litterly represents which objects have been categorized or not. If the object ID isn't in the table then the object hasn't been categorized. I can't check right now but perhaps I'll be able to have a look later.
it looks like tiki_category_objects is the table that actually tells you what category each object is in.
As far as the documentation goes, it's a work in progress, I wouldn't take it as gospel.