I would have said that it was the truth table. Although it could also be talking about the equation. It sounds mis-quoted or missing some extra information to me.
Essentially the truth table and C.E. show the same thing, the operation of the flip-flop. So essentially which you choose is dependent on the situation in which you are using it, unless one is specified.
There is between the current and next state's ouputs and the current inputs, that is you can create one surely - it just isn't of much use for an element with a reasonably simple state table.
But as you say Warren; there isn't an equation OF a flip-flop.