Optional Requests GENERALLY relate to items such as 'Cot in the Room', 'Newspaper in the Morning', 'Dietary Requirements' etc. Whereas as Chargeable Extras are items such as 'Bottle of Champagne in the Room 50€', 'Flowers in the Room 49€', 'Room Upgrade 29€' etc, etc
If the item is set up in Chargeable Extras then the goods are purchased and paid for by the customer directly when they book the room AND ACCUBOOK REFUNDS THE MONEY (Minus the commssion) TO THE HOTEL.
To avoid any commission charges hotels tend to set up ALL items in the Optional Requests area, (ie the customer only pays for the goods when they arrive). The Chargeable Extras section is only really used when the hotel needs to be sure it is going to get its money upfront ie If they have to go and purchase something.
It is worth noting however, that the take up on extras seems to be increased when the goods are offered as a Chargeable Extra, our own assumption is that the customer believes if he is paying for it up front they are guaranteed they are going to get it. This however is all conjecture.