When I book a journey via Greyhound Lines or Jefferson Lines, the offers include journeys by other bus companies, such as Burlington Trailways or Black Hills Stage Lines. This goes both ways; booking via Black Hills Stage Lines also shows trips by Greyhound Lines. The interfaces used by the website search engines appear very similar.
Is there some cooperation of bus companies so that they sell each others tickets?
If yes, who are included? If no, according to what system do I get listings from company Y on the website from company X?
Does it matter where I book?