There is no standardizing organization within canyoneering, so you won't find an "official" rating for slot canyons. I have see three different iterations from different groups, but I will include the most used one here. This is adopted from the American Canyoneers Association.
When it comes to "ratings", what this refers to is the technical difficulty that is found within it. It describes how much water is in there "normally", how difficult the hardest obstacle is, how many hours is required for the approach, canyon and exit, and last, what type of additional risk(!) is in there.
Let me give you a few examples of Canyon Ratings and see if you can decipher it:
The Subway (popular canyon in Zion)
Bluejohn Canyon (where the famous Aron Ralston got stuck for 5 day and cut off his arm):
U-Turn Canyon (popular route in Arches NP)
Neon Canyon (popular route in Escalante-GS)
3B IV R
Lets discuss what these ratings actually signify!
Example: 2A III R
In the first column we have:
Technical Class (Range: Class 1 - Class 4)
Class 1: Non-Technical Canyon Hiking (meaning no ropes required). Check route details for pertinent information.
Class 2: Basic Canyoneering. Easy climbing/down-climbing and a rope may be helpful to assist in hand-lines, lowering backpacks, and aiding others over obstacles. Exit/Climbing out is possible without fixed ropes.
Class 3: Intermediate Canyoneering. Technical climbing/down-climbing and a rope will be required for rappels. Requires basic problem-solving to proceed down-canyon.
Class 4: Expert Canyoneering. Contains all of Class 3 Obstacles and more. Can include multi-pitch rappels, advanced rope-work techniques (which is not taught on Canyoneering101.com), and may contain difficult potholes which may be serious in nature due to water levels, may contain extensive down-climbing, high stemming (which means 20 to 60 feet above the ground and a slip would be disastrous), difficult anchor or rappelling issues, possible long sections of swimming, and rappels that may number 20+ and so efficiency and being quick is key.
The second column is:
Water Rating (Range: A, B, & C.)
Water Rating: "A": Normally dry, or may contain a little water after a rain storm. Possibly, waist deep.
Water Rating: "B": Normally a little current (or still, depending on conditions). Pools are to be expected, as well as swimming.
Water Rating: "C": Normally a strong current flow. Waterfalls. Rope techniques for descending them are required (not taught on Canyoneering101.com).
The third column is:
Time Length/Duration (Range from I to VI)
This combines the approach, canyon, and exit.
Time Rating "I": Short. Requires 1 to 3 hours.
Time Rating "II": Half-Day. Requires 3 to 6 hours.
Time Rating "III": Half-Day to Full-Day. Requires 6 hours to at least 10 hours.
Time Rating "IV": Full-Day. Requires more than 10 hours, but less than 24.
Time Rating "V": Full-Day + 1. Requires more than 1 day to complete.
The fourth (and additional) column:
Additional Risk (These range from No Rating to X).
(No Rating): If there is no rating listed here, than this canyon just has the assumed normal risks.
Rated "R": At least one extraordinary risk may be present. Not for beginners (at all). Falling, drowning, risky anchors/rappels is possible. Solid technical skills MUST be in all individuals.
Rated "X": More than one extraordinary risk is present. In addition to Rated R, Rated X canyons have the real possibility of seriously injuring and possibly death.