Setting a Finish Date at the task level creates a constraint, but not when you change a Finish Date on the assignment level. Is changing Finish Dates at the assignment level "safe"? (i.e. won't cause scheduling issues like changing Finish Date at the task-level would?)
testingthe, Is it "safe", that depends. The only reason I could see for editing an Assignment Start or Finish is to adjust work assignments when more than one resource is assigned to a task. If you are doing it for a task with only one resource I gotta ask, why?
The Constraint Type field is a task field so it responds to entries in the Task Start and Finish fields. Assignments are not tasks.