It’s on Page 13 of the Vivaldi Clock Manual;
The “TTL” (Transistor-Transistor-Logic) is in reference to Square wave reference clock input signal, while the “AC” is in reference to a Sine wave reference clock input signal.
An impedance mismatch of a 50Ohms clock output to the 75Ohms dCS reference clock input is liable to work unpredictably, especially given the relatively high frequency of 10MHz, I expect there’ll be lots of reflections in the signal.