Before answering your question I would like to give you high level of overview how system generate purchase schedule.
The generated purchase schedule (supply) and lines are based on the delivery moments. EP groups all the orders to the nearest delivery moment before the supply with in the Horizon Date. In case no delivery moment before supply date it groups the supply to the immediate delivery moment after the supply.
Now how system would decide the firm requirement or planned requirement for example Current date to till 30 days it would be firm as we define in segment XX with length of 30 days plus system consider the Delivery moments & for planned After 30 days to till 10 weeks it would be Planned as if we define segment length for
segment “XXX” as 10 weeks plus system would consider nearest Delivery moments.
Now so far your question is concern If EDI is implemented than "MRL001" would be sent to your supplier once you print "print purchase release" as per your planed demand by considering segment length and delivery moments and again if there is any change in demand than system regenerate requirement type planed and firm by considering segment length and delivery moments.
We can use the session "Purchase Release line-Details by schedule" to track the Purchase schedule against release type (shipping schedule/Material release) and Release revision.
If any further clarification require or some requirement I misunderstood than please do send me the mail on firstname.lastname@example.org