Please remember that each coil has 2 terminals.
So if one terminal has a dot, the other one won't have.
In CTT, the secondary has 2 coils hence when the transformer is under construction we have 4 terminals at the secondary side first.
Usually we wind these 2 coils simultanuously (2 wires in parallel for example).
So at one end, we have A1 (for coil 1) and A2 (for coil 2).
Obviously at the other end, there will be B1 (coil 1) and B2 (coil 2).
In other words, A1-B1 (wire of coil 1) and A2-B2 (wire of coil 2).
So if A1 has a dot, A2 should have a dot too (B1 and B2 won't have a dot).
This means if Va1 > Vb1 (for coil 1), Va2 > Vb2 (coil 2).
Note please that if we decide to join A1 and A2 also B1 and B2, we would get just one secondary coil formed by 2 wires.
Now it is time to get the 3 terminals at the CTT secondary side:
A1 will become your terminal A (on your pic)
B1 + A2 will be your terminal B
B2 will be your C.
We say here that we connect the two coils (wind in parallel to let them have the same magnetic flux) in series hence from A1 to B2 the voltage is doubled; (Va1-Vb1) + (Va2-Vb2).
On the pic of jony130, there are 2 dots which are in our example denote the terminals A1 (of coil 1) and A2 (of coil 2).
So on your pic if Va > Vb, Vb > Vc. Please note that the dots are about voltages not currents.
I am saying this because if we add a 3rd dot at your terminal E (primary side), when Ve > Vf, it likely means that a current enters the coil from e to f. But at the secondary since Va > Vb a current should go out from A to the load, so the current direction is from B to A inside the coil. So please remember that "dots are better seen as defining the polarity of coil terminal voltages, not the coil current directions".
Kerim
Edited:
Many mistypes are corrected above concerning coil terminal lables... sorry for any inconvenience.