There are different levels of judges and each has requirements they must meet and limitations to what they can judge.
L - (commonly called a "learner" judge)
To enter the "L" program candidates must have either show at 2nd level with scores above 60% or have trained horses or riders to 4th level or higher. There are written and practical tests which must be passed with minimum scores. L judges are usually found at schooling shows.
Recorded - this is the "little r" judge.
Has to have passed the "L" course with distinction, compete consistantly at 4th level and receive at least 12 reccommendations from licensed judges. An "r" judge is licensed by USEF and can judge at licensed competitions through 2nd level.
Registered - "Big R" judge.
Must achieve all requirements for an "r" judge, and hold that positition for at least two years. Big "R" judges are licensed by USEF to judge thru 4th level at licensed shows.
Senior - "S" judge.
Senior is the top ranking available from USEF, they must achieve all requirements for an "R" judge and be reccomended by other "S" judges. They must show profientcy at the FEI levels and are able to judge thru Grand Prix at licensed shows.
Then there are FEI levels of judges too.
International Candidate Judges, International Judges and Official International Judges (these are the "O" judges of which there are only two in the US - Axel Steiner and Linda Zang...oh, is there a third now? Hmmm, I think it's just the two still.)
International Candidate Judges - must speak English and preferable to understand French; must have been a Senior Judge for at least 5 years and have judge thru Grand Prix at CDNs, CDIYs, CDI * or CDI **. They must also have passed some FEI tests and have assisted an "I" or "O" judge 5 times.
International Judge, must have held the position of International Candidate Judge for 3 or more years + additional competitions at CDI ** and CDI ***.
Official International judges "O" are the highest international ranking. Must have been an International Judge and are selected by the Bureau of FEI.