RE: Negative/Positive Days
Do you raise POs more than 100 days in the future?
Positive days is the immediate window in which master planning will assume you want to reserve inventory for demand that is expected to arise within that window For alter demand it will not allocate stock to that future demand but it will trigger a new PO in the future.
To consider actual on-hand for any demand, the positive days have to be at least as long as the coverage fence. Higher than needed positive days generally are not generally a problem - but don't leave it blank or mrp will ignore on hand stock.
Negative days will look to see whether a PO can be brought forward rather than create a new PO. However, it does not know whether that PO can be changed - its a contractual commitment and may have already been shipped by sea. So the system will plan a new order to cover a short term demand, but will also give you a message to recommend you cancel it and instead advance an existing order.
If you are using high numbers like 100, then its very important to review Action messages.
dynamics negative days is generally a better option and is based on lead time.
Negative days then can be considered more as a minor extra tolerance on the dates which you can live with.
Note negative days when used to replenish minimums don't; have a vendor lead time, so will take the default LT from the product master which may differ for a specific vendor or not be realistic for production when the production quantity is high.