Neither Ardour nor JACK cares. Word clock is used to synchronize device on hardware level.
An analogy: If you read the time on your wristwatch, you don’t have to know how that timekeeping works under the hood. You just take it for granted. JACK does the same.
If you use two soundcards, you’ll want them to be in sync. This is like synchronizing two clocks.
When you have a soundcards running at, say, 44100 samples/second. It is never exactly 44000 samples/sec. It may be 44100.001. Now if you have a second soundcard it may sample at 44099.997 samples/seconds. Over time they will drift apart.
The solution is to send “clock pulses” from one device (the master) to another. So the devices can synchronize their sample-clock and update in lock-step. This prevents drift.
A 2nd consideration is to pick the “master clock”:
Some cheap devices have a poor internal clock, that jitters, or is always a bit to slow/fast depending on temperature, power-supply or whatnot. In that case you want to slave this device to a steady clocks-source from a reliable master-clock.
I don’t know any of the devices you mention, but apparently the persons you spoke to suggest to use the Rosetta 800 as master, and have other device use the clock from it.
After that you start jackd using the rosetta as main device, and then add the other using
alsa_out or zita-ajbridge’s