Project / Task
The Project (and Task) fields in conjunction with a person's association with an accrual plan, determine which project (and possibly task) the user will be reporting their time against (for this particular association). Hours accrued will be deposited on an assignment for this project / task.
Note that if the project is defined as requiring a task, yet has no active tasks, you will not be allowed to select that project when making the association.
As a best practice, if you are capturing your accrued time at the project level (rather than task level), it is best to ensure the project you are using has no active tasks defined (otherwise, users may charge against those sub-tasks, but that will not count against the accrued budgets as they would be stored at the project level).