Purpose:

Edit the default due date, invoice subject, and invoice notes that will appear on a new invoice.

Explanation: 

An invoice (sales invoice) is a document requesting payment for products and services. In this case, a sales invoice is applied
to Customers of the Cheqbook user. The invoice usually includes the products/services, quantities, prices, and payment terms. The default values for due date, subject and note will appear on the invoice when you create a new invoice. The default values can be over written before you save the invoice.

Actions:

Enter Due Date: Select due date from the "Default Due Date" drop down menu.

Enter Subject: Write the default subject in the "Default Invoice Subject" field.

Enter Note: Write a default note in the "Default Invoice Note" field.

Save: Click "Save" to save your changes.

Cancel: Click "Cancel" to cancel your changes.

Additional Resources:

Wikipedia Invoice