Customers and Account Codes are lists that can be associated with Project and Expense entries for reporting purposes. Associating Projects to Customers is handy for invoicing. Associating Projects to Account Codes can act as a subtask of a Project.
Customers and Account Codes should be set up in advance and then selected on Project and Expense entries when the time is entered or edited.
You can turn Projects and Customers on and off via the first page of the Company Settings. You can add and edit your Projects, Customers, and Account Codes from the left menu. With each, you can restrict how they are shared between each other and with other users.