Number of calendar days from the system date inside of which MRP will not create or change existing planned orders. Order action messages, however, will be generated if necessary. The time fence is where manual control is exerted in computer-assisted master scheduling

The MRP horizon should be at least as long as the master scheduled item with the longest cumulative lead time for the item being scheduled or else key materials may not be planned to be available when required. MRP only processes material requirements within this planning horizon

Order Release Horizon is the number of days MRP looks to the future to see when orders are due to be released. In the screen shot the value is seven days. MRP will start creating action messages “order due for release” seven days before the release date.