How does switch matrix card PXI 2535 work without GND?

Hi guys,

I am using switch matrix PXI 2535 for continuity testing. When i see the datasheet i found that there is no GND reference, i mean there is no GND from connector. can anyone tell me how does the card work and what would be the GND reference for the instrument is it Tester GND or anything else?

Regards,

Alekhya