The resonance frequency in Selyn in an individual that has distinctive characteristics.
The following is the understanding of a Companion and Channel not trained in the level of transfer mechanics science known at the time of Zelerod's Doom ( Year 0 Unity), struggling to explain what is felt as true to those who do not have any way of knowing. When reading an utterance of a character, take into account what year they are speaking in, to whom they are speaking, and their level of education in their subject. How a character explains something to another character is framed to characterize both and move the plot forward, not to instruct the reader in what is actually going on.
From: Zelerod's Doom, Ch 6. An explanation from Sergi ambrov Keon; "It's a special relationship between a channel and a Companion who are matchmates," Sergi explained. Then he had to explain that Simes and Gens had frequencies of selyn resonance as distinctive as facial characteristics. Matchmates were people whose frequencies and capacities were so similar that they were irresistibly drawn to one another."
From: Zelerod's Doom, Ch 6. And from Risa Tigue Ambrov Keon; "In a channel and a Companion,"Risa took up the explanation, "those forces are very powerful, and when two people work together and have transfer together, they...adjust until their resonances...harmonize. But it's not only physical; caring deeply for one another also affects their fields. And the result can be lortuen or torluen."
Unknown to this pair at the time when Unity had barely been imagined, this quality of Resonance would become the basis of an interstellar spaceship's drive, a technology some historians would eventually credit with humanity's survival against all odds.