The bookkeeping "account codes" or "chart of accounts" will make it easier for you and your accountant to recognize and track certain income and expenses as different Accommodation types or Extras, in the bookkeeping report.
Your accountant might prefer a certain way of recording income and expenses, but otherwise you might just write CASH, TAX, DEPO (Deposit), ROOM, BREAKFAST, RENTAL, etc., in each field. You can add these names or codes for each Accommodation type and Extras, by going to "Settings > Payment & Invoicing > Chart of accounts".
You can create and view a PDF bookkeeping report showing the total for each account at "Invoices > Accounting report".
The bookkeeping system is based on the double-entry bookkeeping system which is a set of rules for recording financial information in a financial accounting system. This entry in Wikipedia explains it more fully: http://en.wikipedia.org/wiki/D