Have you tried the search function in this forum? There are plenty of threads on it. Also SAP help is there if required.
Please refrain from posting queries which have been asked.
However in SMK if you face any specific issue which is not discussed earlier, do post n am sure the forum will be of help.
Schedule Margin Key is nothing but Floats required for Scheduling the Order.
Normally It consists of Opening Period, Float Before Production, Float after production and Release periods.
1. Opening Period: The number of working days between the date that the order is created and the planned start date.
2. Float Before Production:The number of working days between the order start date and the scheduled start date; used
as a float in production scheduling.
3. Float After Production:The number of working days between the scheduled finish date and the order finish date; used as
a float in production scheduling.
4. Release Period:The number of workdays between the planned start date of the production order and the date for
releasing the order.
FYI I guess there are so many threads related to this query,kindly please search in this forum for more info and also check these