Requirements are what have to be done to achieve a Goal.
-In order for Sam to live (Goal) he must have a surgery (Requirement)
Prerequisites are the things that need to be done before the Requirement can be brought about. They’re like requirements for the Requirement-or Prerequirements, if you will.
-Before Sam can get the surgery (Requirement), he must lose weight (Prerequisite).
Preconditions are about getting the surrounding conditions right to bring about the Requirement. They are conditions that must be in place prior to achieving the Requirement.
-Before the doctor will perform the surgery (Requirement), Sam must have sufficient insurance coverage to pay for the procedure (Precondition).