A Deadline is the date by which an item must be completed.
Whereas a Plan is your personal goal for when you plan to do something, you can use a Deadline to keep track of when it must be completed.
For example, imagine you have an essay that must be turned in by next Friday. You might set the Deadline for next Friday, but the Plan for sometime this weekend - helping you track the difference between when you plan to actually write the essay, in contrast to when it must actually be completed.