Next, we see a set of enumerated types:

enum LetterStatus {
  o AWAITING_APPROVAL
  o APPROVED
  o SHIPPED
  o RECEIVED
  o READY_FOR_PAYMENT
  o CLOSED
  o REJECTED
}

These are the states through which the letter is going to transition. When we access a letter, we're going to be able to identify where the business process is using this enumeration. All the names are fairly self explanatory.