Unconstrained and Earliest date

Hi ,
I can understand the concepts of Unconstrained Earliest Possible Start Time, The Unconstrained Earliest Possible Completion Time,Unconstrained Latest Possible Start Time, Unconstrained Latest Possible Completion Time, earliest possible start time, earliest possible completion time, earliest order date, earliest completion date, earliest demand satisfied date. But i can get the concept of earliest allowable completion time. Can you please explain me the function and also how i can use all these information in analyzing the output of the ECC plan.

Also can you explain me when does  the exception message requirement and order lead time constraints are raised in a ECC plan.