1What do I need to get started
2General questions
3How much will I get paid
4How music is published
5Digital stores
6Legal questions
7Music Promotion
What is iTunes Pre-Order?
This is option allow make pre-order on iTunes, before release date. 
There is no support for a pre-order on singles.  Pre-orders are only for products with 4+ tracks and you should to submit album from 5+ days and more...