Where the schedule margin key is customized?

You define the order float in the scheduling margin key, The scheduling margin key is copied from the material master. The float before production or float after production and the release period are defined in the scheduling margin key

For Customizing for Shop Floor Control, by choosing Operations --> Scheduling --> Define Scheduling Margin Key .
The scheduling margin key is assigned to the material ( MRP area in the material master) and is transferred when the production order is created. These values can be changed in the production order.