In this place you can customize text on quotation. You can also set numeration format and enter some messages displayed only on quotation.
Numeration rules for Quotation are the same like for regular invoice. Read more in invoice settings.
Quotation can be easily converted to invoice. Just select your quotation on list and click right mouse button to show hidden menu. Select Convert to Invoice option. It will open invoice details dialog with new invoice but with items and totals based on selected quotation.