The BOM selection logic is quite complex as several customizations and fields of the material master are involved, so I am summarizing it here.
First of all, the BOMs of a material may exist in the architecture below.
1. Usage: e.g. production BOM, costing BOM, engineering BOM etc.
2. Multiple BOM: Multiple alternative BOMs under one usage.
3. BOM change status: Different BOM items/quantities etc in different periods which can be made by change numbers.
Please see the picture below.
The BOM determination in MRP is done level by level through the structure above and by the procedure below.
Here is the detail of the steps.
Step 1: Determine the BOM usage.
The usage is determined by the selection ID which can be configured in customizations below. In the MRP customization, the selection ID to be used is defined on plant level and the BOM customization OS31 defines the priorities of each BOM usage. If there is no BOM exists under the usage of the first priority, the usage of the next priority is checked.
Step 2: Determine a BOM in the multiple alternative BOMs.
This is decided by the field 'Selection method' of the material master in the picture below.
2.1 Selection method is blank (Selection by order quantity):
The quantity range of each BOM header is checked to see which BOM can be applied according to the order quantity, here is a picture for an example.
Here is the screen shot of the quantity range of multiple BOMs.
2.2 Selection method is 1 (Selection by the explosion date):
To use this method, the customization of the picture below should be maintained.
In the example above, the alternative BOM 1 is valid from May 1 to May 31 and the BOM 2 is valid since June 1. They are selected by the explosion date, either the order start date or the end date or the date of the explosion number according to the customization transaction OMIW in the picture below.
The one of the MRP group has higher priority.
2.3 Selection method is 2 or 3 (Selection by production version):
The production version can be maintained in the material master and it defines both the BOM usage and alternative BOM as the picture below.
Step 3: Determine the valid BOM according to the BOM change status.
A valid BOM is determined by the explosion date in multiple periods of an alternative BOM as shows in the picture below, the periods can be made by change numbers.
The explosion date is the one explained in the step 2.2 and here is more information of the date of explosion number. Using the explosion number all levels of BOM are exploded by the same date of the explosion number as showed in the picture below instead of the start or the end date of planned orders in each level.
Here is the explosion number maintenance transactin MDSP and the fields where the explosion number can be entered in the PIR (Planned Independent Requirements) and sales order.
Additional checking points:
1. BOM header
2. BOM Item:
The BOM item should be relevant to production.
3. Customization of specific BOM explosion:
The field 'Excl.Bulk Mat.' controls if the bulk materials can be exploded.
The field 'Sales Order/WBS' can check whether the sales order BOMs or WBS BOMs can be exploded if they have.
4. BOM buffering:
To improve the performance of MRP, the exploded BOM can be buffered and MRP reads that onward instead of reading from tables unless the BOM is changed. Please pay attention to the statements below in the F1 help of the field above.
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Note that you should not switch off this indicator and then switch it on again a short time later, because then the BOM time stamps are no longer current. In this case, the wrong BOMs could be used. However, you can delete the buffered BOMs in the shared buffer using the report RMMDBOM1.
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For more information, please refer to blog Problems with BOM explosion on MRP (Exception message 52).