Borella is the planet from which the Borellian Nomen Bora, Maga, and Taba originate. When serving meals aboard the Prison Barge's mess, Baltar admits that the Borellian desert layers he saved for the Nomen were not actually from Borella, but were the closest thing available from the livestock ship (TOS: "Baltar's Escape").
The Nomen describe Borella as "the land of the mega sun and the endless sands" (TOS: "The Man with Nine Lives").