The best explanation of the MRP Recalc Needed process can be found in the MRP Technical Reference. You can get access to the PDF Technical Reference Guides via the Application Help. Here is an excerpt:
The MRP Recalculation Needed is a calculation method that lets you reduce the need to run the MRP engine in Regenerative mode. This mode deletes all previously generated MRP information in order to calculate new job, transfer order, and purchase suggestions.
Constantly running the MRP engine in Regenerative mode can strain your network resources, which can cause the MRP process to run slowly.
The MRP Recalculation Needed method provides better results than when you run the MRP engine in Net Change mode. Net Change mode restricts the amount of calculations run by the MRP engine, because it ignores any previously generated information. When you use the MRP Recalculation Needed method, however, demand is recalculated for records that are typically ignored by Net Change mode.
For example, suppose that no activity is placed against a sales order when the MRP engine is first run. If the sales order has a future Due Date that is outside the Cut Off Date range, the sales order is recorded by the MRP engine, but it does not create any suggestions against it. When MRP processing is run in Net Change mode, no activity will be placed against the sales order, because its values were already recorded by the Process MRP program. Thus, the sales order is completely ignored by MRP.
The MRP Recalculation Needed method makes sure that all the sales orders are correctly evaluated by the MRP engine. Because it eliminates the need to do a full regeneration every time MRP is run, this calculation improves efficiency. This is because the MRP Recalucation Needed process does not review a part’s Reschedule In, Reschedule Out, or Planning Time Fence date ranges. If no other changes are made to records linked to this part, then this part is ignored during the Net Change MRP processing run.
Note that you can set up this calculation method to run automatically on a recurring schedule. If you process MRP automatically, you should set up the MRP Recalculation Needed process to run just before the MRP process. This maximizes effeciency during the MRP process.
In the Technical Reference Guide there is additional information, such as the modifiers for the process, Logic/Algorithms and an Example Business Case.
Just search the PDF for ‘MRP Recalculation Needed’ should be around page 108.