Schedule vs. Due Date - Short summary

due date

when the to-do must be completed. After this date, the to-do is overdue. Overdue items are visible in the Today list, and are shown in red. Use due date to track to-dos against real world deadlines.

scheduled date

when you want to start tracking the task. After this date, it's active and is displayed in Next and Today lists. Until this date, it's inactive and is only shown in the Scheduled list. Use scheduled date to arrange to-dos against user priorities, deferring their activity but not forgetting to track them.

[[NB: This is my preliminary understanding of the terms. Please correct any errors.]]