The term "AES sync" refers to the use of an AES3 digital audio signal as a clock source of synchronization in digital audio systems.
The AES3 digital audio format is "self-clocking" in that the same signal carries both an embedded clock and the digital data. This allows for the use of a single connection to carry stereo digital audio form one device to another without the need for any other form of "clock" connection.
The AES3 standard specifies connections with both 110 Ohm XLR/twisted pair cables and 75 Ohm BNC/coaxial cables. Most AES sync connections take the BNC form; and thus have a 75 Ohm termination. For short cable lengths; the difference in impedance can be ignored and a simple wired adapter or adapter cable used to adapt an XLR source to the BNC input.