I'm not sure there is a hard and fast rule, somewhere in there it transitions to a plate as well.. but where to draw the line seems a bit arbitrary depending on all the proportions of the piece.
I guess if it walks like a duck and quacks like a duck we can call it a duck even if its a robotic killing machine bent on world domination. Or if it looks like a bowl, works like a bowl then you could still call it a plate if that sold better... I suppose.
Love thy neighbor, yet pull down not thy hedge.