Customer Analysis Fields allow you to note any extra information for each Customer that is not automatically collected by Spoonfed.
For example, some Customers might prefer to have a separate Invoice for each Order whilst others would prefer Multi Order Invoices. You could set up a Drop Down field with the options 'Single Order' or 'Multi Order', and then search by preference in the Invoice module.
For Date fields, you might want to record the Date each Customer started Ordering with you, and message them on their anniversaries.
An example of a Text field might be to keep note of their loyalty card number, if you run a loyalty scheme.