What is the difference between a proforma and an invoice?
In short
A proforma is only a request for payment, not an invoice, so it is not reported to NAV; an invoice documents the supply (or an advance received). If an advance arrives on a proforma, the VAT Act requires an invoice for it.
Updated:

Side by side
| Proforma | Invoice | |
|---|---|---|
| A tax document? | No | Yes |
| Reported to NAV | No | Yes, as a rule |
| When | Before payment | On supply, or when an advance is received |
This is general information, not tax advice; check your case with your accountant.
Which module?
Module: Invoicing (paid module, price coming soon).
Where is the full guide?
This is the short answer. The longer guide walks through the steps, the pitfalls and the examples:
Questions
Can HelloCRM issue a proforma?
Yes, in the Invoicing module; proformas are never sent for reporting.
Sources
This is general information, not legal or tax advice. The rules come from: 2007. évi CXXVII. törvény az általános forgalmi adóról (Áfa tv.), Nemzeti Jogszabálytár (checked ); NAV Online Számla (checked )