When the Use sprints for manual scheduling option is selected, tasks can scheduled to begin and end based on sprint dates.
When sprints are used for manual scheduling, tasks that are assigned to sprints will be scheduled for the duration of those sprints, based on current sprint dates and future sprint estimates. The work for each task will be evenly distributed across its sprint duration. For more information about sprint-based scheduling, see Planning with Sprints.
Prefer sprints over manual start and finish dates
If this option is selected, issues that are assigned to a sprint will automatically be scheduled based on the sprint. If this option is not selected, issues with manual Start or Finish dates will be scheduled based on those, regardless of their sprint assignment.
To learn more, see Scheduling Precedence.