From the song "Hitting on All Sevens" we get a peak into the caste system in Heaven. If you notice in the trailers each caste has an arm band with a number, to show which they are in. Also the different floors on the Devil's Carnival website, going up to heaven also correlate with the castes. They go as follows from the song:

Caste 1 - Kept as pets - the Agent

Caste 2 - Steer the Flock - the Librarian

Caste 3 - Plumes and Pelts - the Designer

Caste 4 - Birds of Song - the Publicist and his Ladies of Virtue

Caste 5 - Watching Dogs - the Watchword

Caste 6 - Preying Beasts - Translators

Caste 7 - Working Horses - Fillies and Glad hands

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