The business transaction processing system we are building doesn't know anything about state transitions, but just keeps track of numerical functions of time like the value of an account. A complete system requires each model, and so each software package, to work with the others. The two main kinds of objects in Accounts are transactions , which are objects like invoices, checks, or timecards, and accounts , which keep track of the status of some business object like an inventory item, a vender, or an employee. Transactions model the pieces of paper that flow between businesses, usually describing either the flow of money and goods or commitments of money and goods. Thus, they describe how much inventory you have, how much you owe your venders, and your net worth.
Transactions are fairly simple, basically a record with a date.
Accounts are much more complex, since their attributes change over time. You don't ask for the amount you owe a vendor, you must ask for the amount that you owe a vendor at a particular point in time, because the amount you owe changes from day to day. Accounts transactions are related to database transactions, but are not the same, because a single Accounts transaction might turn into several database transactions. In the same way, an account in Accounts can model an account in an accounting system, but is not the same. In Accounts , an account can have many values.