Hi Izago,
The digital operating modes on the AD6655 (NCOs, FIR filters, etc) are available in either CMOS or LVDS mode. The AD6655 has the two output mode (LVDS or CMOS) for flexibility in designs. When this part was first released there were many designs still employing CMOS interfaces and some moving to LVDS interfaces so the device was designed to be compatible with either interface.
Regards,
Jonathan