Exactly, so today's email client broadly serves two functions: 1) to manage emails (which is to say, manage the flow of tasks that emails represent), 2) be a knowledge base.

Reference labels are essentially meta data - so you might create a general reference labels for invoices, receipts, meeting notes, etc. You can then use ActiveInbox's sidebar menu to drill into the knowledge base -> look for meeting notes for a given project, or invoices for a given contact.