Use Sprints for Manual Scheduling
When the Use sprints for manual scheduling option is selected, tasks can be 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.
Issues that are not assigned to a sprint (and do not have manual Start or Finish dates) will be represented on the chart in the Backlog panel. You can assign those issues to sprints by dragging them onto the chart or using the Task Details Panel. When you do, Structure.Gantt will display sprints from the selected board.
You can only select one board for the backlog. If your chart includes issues that need to be assigned to sprints from another board, you can assign specific boards for each project using Slices.