In *Planet of Exile* Agat says that his people learned mindspeech from "another race, long ago, on a world called Rokanan."

In *Four Ways to Forgiveness* Solly had learned "farthinking from an old hilfer on Rokanan" which is presumably the same world.

So Rokanan is a place where the inhabitants know, and can teach, mindspeech (telepathy).

In "Vaster than Empires and More Slow" we have "one of the recently discovered worlds, a hilfer name Rocannon reporting what appears to be a teachable telepathic technique"

In *Rocannon's World* Rocannon is referred to as "Lord Rokanan" by the inhabitants.

Putting that together, we have Rocannon discovering a world where telepathy can be taught, and that world later comes to be called Rokanan.