What do the different tiers mean?

[Mach7]

On the PDGA website the tournaments are listed with many different tiers: A, B, C, XC, B/C, NT etc. What do the different tiers mean? I realize that an A level tourney is a higher caliber tournament than and B level, but are there any rules/definitions set around the different tier types?

Much thanks!

 

[Vegan Ray]

http://www.pdga.com/files/documents/TourStandards.pdf

 

[Mach7]

Thanks!