“what does the AS SYSLINK cable do? ”
The AS-SysLink cable allows two units to share an external GPS Receiver. Either unit can power the GPS Receiver so either one or both can be on at the same time. There is no sharing of any data between the units: they only share the NMEA output from the GPS Receiver – nothing else.
“Can you eliminate the interlink with this cable”
No you cannot eliminate the InterLink by using this cable as the cable does not allow for the sharing of waypoints or Routes between the units like the InterLink does.
“Can you run from a 798 with internal gps to a 728 for example?”
You could but it would mean cutting and splicing the AS-SysLink cable, not just plugging both units up as is.