Only devices that are Roon Ready or Roon Tested are capable of being identified by Roon. Neither the DACmagic100 nor the iFi DSD Pro is certified, because of that they won’t be recognized by Roon.
In some cases choosing a device from the drop-down list can result in some features of the device becoming inoperable. Generally, we don’t recommend misidentifying devices in Roon. If you haven’t detected any downside to doing so you’re free to give it a shot.
What does Roon do when it can’t identify a device, but can identify another device from the same manufacturer?
It’s been awhile since I connected my DAC, but (as I remember) Roon incorrectly used a Device Setup from another iFi DAC. I kept that setup because it worked. I don’t remember picking from any drop down.