On below, you can see the explanation of Rescheduling Horizon made by SAP themselves. This is mostly confused with Replenishment Lead Time.
Planning Time Fence
If you setup planning time fence and if this planning time fence is over rescheduling horizon; at the same time you have fixed quantities within planning time fence, even stock level goes below zero, your new proposals won’t have exception messages.
Because MRP will only provide you exceptions for the proposals within rescheduling range. stock/requirements list (MD04) considers rest as irrelevant. So, MRP list (MD05) will not consider related time span.
So you’ll see exceptions after fence in MD05 and not in MD04 under same setup.