When you edit a client, there are a number of fields intended to store information about invoicing.
These fields include Default payment term, which specifies how invoice due date is determined, and Invoicing threshold field, which allows to set a limit, upon reaching of which the system will offer to create an invoice for a customer. Also, Comments field allows to add a free text describing invoicing process for the particular client.
In addition to this data, each customer's contact has an Invoicing matters checkbox, which tells the system that this person may be contacted for invoicing matters. Such, there could be a number of invoicing emails for each client. In this case, when you intend to send an e-mail, these contacts will be automatically added to the To filed of any composed message. Of course, it is possible to edit them before actually sending an e-mail.
A dozen of settings which affect invoice creation process can be found in the program Settings, on the Invoice settings tab. Here you can setup default invoice file names, default tax used in a newly created invoices, different numbering options, logo for your invoices and signature text. Also, information about unpaid invoices can be added to a Google Calendar.