Preno will take each item of a reservation (accommodation rates and extras) and put them on their own line item on the invoice. If you've mapped your chart of accounts correctly in Preno, they'll also be sent through to different revenue accounts.
The description for a rate is defined by the name of the rate in Preno. It will go through to an Accommodation Revenue account. The rate is sent through as the unit price, and the number of nights is sent through as the quantity. Multi-rate bookings will have multiple line items for rates.
The description for an extra is defined by the name of the extra when it was added in Preno. It will go through to an Extra Sales Revenue account. The price of the extra is sent through as the unit price, and the number of extras sold will be sent through as the quantity.