whats the difference i dont get it

D tiers are developmental and are usually smaller events and the only PDGA event at that venue. C tier is the normal competitive event. B tier adds a points multiplier for PDGA members. It also requires a player pack. Pro B tiers have payout minimums and "added" cash minimums. A tiers minimums are higher. Majors are usually invitation or some kind of qualification to play.

ok thanks

What about tier-X and...'RC'[?]?

X-tier is usually an experimental format sanctioning level. It most frequently is seen for doubles competition although there are no official PDGA rules governing doubles - hence the experimental tag. The same is true for other specially modified tournament formats such as Teams competition which utilizes a "skins" type format. Also, The Moffitt Show in Houston utilizes a 5-card stroke format for a little added fun.

RC-tier is a ratings based format. All players, regardless of their PDGA declared (or eligible) division, are grouped solely by skill level into appropriate divisions. I'm not exactly sure, but I believe it might also be a "trophy only" type event. (Is that true Nick?)

There are official rules for doubles. http://www.pdga.com/rules/doubles.php

The main thing to know about tiers is, if you are not a member, you cannot play A, NT and M tier tournaments.