Difference between Registration Confirmations, Pro Forma Invoices and Invoices

What is the difference between "Registration Confirmations", "Pro Forma Invoices" and "Invoices"?

(A German version of this article is available.)

ConfTool Pro provides several types of confirmations for participants (please also see image 1):

• Pro Forma Invoice: Pro Forma Invoices are preliminary invoices without an invoice number. The Pro Forma Invoice will be converted into a normal invoice once the payment has been registered in the ConfTool system or the registration has been confirmed manually by the organizers.
  Normally this function is not enabled, you can enable Pro Forma Invoices here:
  Overview => Settings => Main Settings for Participant Registration
  Go to the section "Invoices and Taxes" and then enable the option "Issue Pro Forma Invoices First?" (see image 2).
• Invoice: Regular Invoices (also called "bill" or "tab" and labelled "Invoice and Registration Confirmation" in ConfTool) are issued when a fee is charged for the registration for participation and the Pro Forma Invoice option is disabled (or - if enabled - a payment has been registered in the system, see above).
  Invoices are issued by a seller to a buyer, relate to a sales transaction and indicate the services/products, quantities, and agreed prices the seller has provided to the buyer. Invoices are usually the most important document of a commercial transaction and required by the buyer to start the payment process at their organization/company/university, needed to reclaim money from their organization and/or necessary for tax reasons. See also: Invoice - Wikipedia.
• Registration Confirmation: A registration confirmation confirms that a person has completed his/her registration and it is issued if the person does not have to pay a participation fee. This is usually the case for events free of charge or invited participants.
• Payment Receipt: A receipt confirms that a participant has paid his/her participation fees. It is available after the payment was recorded in the ConfTool system (see image 3).
  You have to enable participants' access to receipts here:
  Overview => Settings => Main Settings for Participant Registration
• Confirmation for Participation: This document confirms that a person was present at the event (and e.g. has presented a contribution). This type of confirmation should be issued after the event and is (usually) availabe to every person who has attended at the event (see image 3).