Index -> Interface -> Magnify Window -> Collector Information
In the lower right corner of every Star Chamber card, you'll find information to help you identify its rarity and the set it comes from. The first number tells you which set the card comes from (for example, 4 is The Atok Incident). The letter tells you the rarity of the card (R is rare, U is uncommon, C is common, and S is a card found only in starter decks.) Last is the number for that card in the set.
Some cards have a P for their rarity, marking a special Promo card separate from (but related to) a normal expansion set. These cards are made available in a variety of ways, such as rewards for high-level tournaments. These cards go "out-of-print" when a new expansion is released.