I'm currently on holiday. I wish I could give you a simple answer other than' it depends'. Its a big subject and really needs an extended training course not a forum answer.
Master planning exectutes 4 distinct processes which you can see in the session log for a given plan by navigating to “Master planning”–>”Setup”–>”Plans”–>”Master plans” and clicking the “Session log” button.
In the first step = ‘Update”, is when planning prepares for the second step to calculate coverage. For regeneration it first clears out the InventSumLogTTS records. Update also deletes the entire existing requirement profile, (except for approved planned orders) and then generates a new, as yet uncovered requirement profile for the item and inserts the details into ReqTrans.
The second step, “Coverage”, creates planned orders based solely on the existing requirement profile) i.e. not on any other planned orders it has created during the run, some of which might even be auto firmed).
Factors like BOM level impact which items are considered first, (for formulas alternative items play a part), order type then determine that type of order. and planning policy calendars, lead times and positive and negative days setting, time fences etc impact quantities and dates.
The come the action and future messages steps. Unfortunately many Ax users do not understand and thus do not use messages. They are not always as helpful as they could be and they extend the processing time. In most other mrp systems I have used messages are the key output and planned Release order messages are just one type of message -i.e. in Ax terms Planned orders. These are not just messages. Up to this stage, AX only used the calculated requirement profile to generate planned orders. Without action messages enabled, this basic coverage plan is what the user sees and no messages are calculated. When messages are enabled, though, AX runs through the newly created planned orders and gives suggestions on how to improve the plan and that may include cancel a planned order. After the actions are calculated, futures are then calculated based on the created action messages (based on the parameters enabled).
Then you can look at the Explanation tree.
MRP believes what you tell it- but some times what you tell it is not consistent- the lead time is 3 days but I promised the order for tomorrow - messages draw your attention to such conflicts and guide you to resolve i.e. to expedite, redate, etc. Tools like forecasting, ATP And CATP, and parameters like freeze fences, or positive and negative days, and safety stocks, and mrp planning policies, help to reduce the real world noise of such inconsistencies but here will always be things the master scheduler knows that mrp doesn't and it not feasible to adjust all the parameters daily That's where S@OP planning, ABC Analysis, and messages and an empowered master scheduler play important roles .
You also have to consider how many mrp iterations are needed e.g.
- for inter-company master planning
- or for level by level planning - if you change planned order dates or quantities for end items then how does that impact the planned orders already created for sub-assemblies and components? i.e. after your firm the first level do you need to run mrp again based on those firmed orders to generate new planned orders for the next level? There no standard answers, the approach adopted depends on many factors.
I'm not sure I have answered your question - but maybe it will help you test out your specific scenarios.
