The limiting factor is the number of available CP s. The XP process begins with the Business writing stories on index cards. A story is something the system needs to do. Development then estimates the time required to implement each story. The Business selects the subset of stories that will form the next release and Development makes a delivery commitment.
Development splits each of the stories into a number of tasks.
Each developer accepts responsibility for a set of tasks. The enormous difference between XP and FDD is FDD's additional development of an overall domain object model. As developers learn of requirements they start forming mental images of the system, making assumptions and estimating on that basis. Developing an overall domain object model forces those assumptions out into the open, misunderstandings are resolved and a more complete, common understanding is formed. The domain object model provides an overall shape to which to add function, feature by feature.