The Vdc Specification of a transformer
Transformers inherently put out AC, not DC.  The DC component coming out of a transformer is guaranteed to be zero.

Voltages for transformer windings are RMS voltages unless something else is specifically stated.  This transformer has a split primary so that you can put the two halves in series to drive it with 230 V AC or in parallel to drive it with 115 V AC.  A similar thing is going on with the secondary.  When the primary is driven as described, each secondary winding puts out 6.3 V.  These can be combined in parallel to get twice the current or in series to get twice the voltages.

Let's say you configure the secondaries to get 6.3 V, which is RMS.  Assuming a sine wave, the peak will be sqrt(2) higher than the RMS level, or 8.9 V.  The output will therefore swing between -8.9 V and +8.9 V.  How that maps to "DC" depends on the circuit that does the conversion.  If you connect this to a full wave bridge, for example, then you get the absolute value of the AC wave minus two diode drops.  If the full wave bridge is 4 normal silicon diodes, then figure each diode drops 700 mV for a total loss of 1.4 V.  The DC output will therefore peak at 7.5 V twice per cycle.

Even that is not the final answer.  If you put a cap on the output of the full wave bridge but leave it otherwise unloaded, then it will go to 7.5 V and stay there.  However, if you use that as a power supply and draw some current from it, then the current will come from the cap in between when it is charged to 7.5 V at the peaks of the AC waveform.  If your input power is at 60 Hz, then these peaks will occur every 8.3 ms.  How much the voltage on the cap drops during the 8.3 ms interval until it is recharged again depends on the amount of current that is drawn and the size of the cap.  The effective average DC voltage therefore depends not only on the transformer but on the size of the cap and the current load.