what is thedifference between cancelling , dropping , and withrwaing an enrollment ?

Hi there,

i am trying to understand the difference between:

• cancelling an enrollment
• dropping an enrollment
• withdrawing an enrollment.

from what i read in People Books , dropping and cancelling are the same thing. both of them delete the enrollment from student record, but if this is the case , why do we have  setup  for :

• drop  and withdraw in Grading Scheme table
• cancel dates and withdraw dates in academic calendar.

Thanks