Cash()
A cash transaction may split and merge money represented by a set of (issuer, depositRef) pairs, across multiple input and output states. Imagine a Bitcoin transaction but in which all UTXOs had a colour (a blend of issuer+depositRef) and you couldn't merge outputs of two colours together, but you COULD put them in the same transaction.
The goal of this design is to ensure that money can be withdrawn from the ledger easily: if you receive some money via this contract, you always know where to go in order to extract it from the R3 ledger, no matter how many hands it has passed through in the intervening time.
At the same time, other contracts that just want money and don't care much who is currently holding it in their vaults can ignore the issuer/depositRefs and just examine the amount fields.