Simple Timestamps versus Scheduled and Deadline

In org there are two ways of adding a date to a task:

• Simple timestamp
  
• Keyword + timestamp
  
  • SCHEDULED:
    
  • DEADLINE:
    
  • These immediately follow the task they refer to
    

When do you use one or the other?

DEADLINE

• A DEADLINE is a task which must be finished BY a certain date.
  
• The task appears in Org Agenda on the date assigned for it.
  
• You can get org to alert you to the approaching deadline.
  
• Note: The deadline will always appear on today's date (provided today's date is 14 days or less than the date of the deadline) and on the date of the deadline. To change the number of days for the warning to appear: Customize the variable org-deadline-warning-days.
  
• Customize this according to work style. I work from Monday to Friday and I don't want my DEADLINES appearing for too long periods. So I set the variable to 5 days (setq org-deadline-warning-days 5)
  

SCHEDULED

• These are used for complex tasks that will require more than 1 day to complete. The SCHEDULED date is the date on which the task BEGINS.
  
• The key idea here is "start working".
  
• This is an unfortunate term because people think it refers to the way you schedule an appointment. But an appointment takes place AT a certain time and concludes a short time later. It is not a task performed over several days/weeks.
  NB: To schedule an appointment use a simple timestamp, not the SCHEDULED keyword.
  
• Scheduled tasks appear on the day they start and every day after until the task is completed.
  

Note: One can use repeaters with both SCHEDULED, DEADLINE and also with simple timestamps.