The "Rainbow Six Siege" competitive circuit revolves around the Six Invitational. However, the best way to get to the Six Invitational is to have great performances at the Majors.
Why is a Major called a Major?
The term "Major" is colloquially used in esports terminology to denote a tournament that has been organized by the publisher of the game. Often, game publishers will contract their tournaments out to independent tournament organizers, like ESL or FACEIT.
How do Majors work in 'Siege'?
In "Rainbow Six", tournament organizers are a part of the scene, but the scene is still primarily run by the Ubisoft esports team. Therefore, performance in the joint-organized regional leagues (APAC North, APAC South, EUL, NAL, and LATAM) qualifies you for Majors, as Ubisoft still has some level of ownership over the leagues.
There are three "stages" that feed into the three Majors that occur per year. One happens in May, one happens in August, and the final Major of the year happens in November.
"SI Points" are earned at the Majors and for placement in the regional leagues. The 16 teams with the most SI Pointsat the end of the year, regardless of region, qualify for the Six Invitational, usually held in February.