Hello,
Read SAP note
78867 - MD01: Documentation on the planning mode
1. The order proposals are reactivated, that is, neither bill of material (BOM) explosions nor reschedulings occur (planning mode '1').
2. For the order proposals, the bills of material are reexploded. If a planned order has already been subjected to lead time scheduling, it is scheduled again, although the 'Lead time scheduling' parameter is not used.
3. The order proposals are created again from scratch (planning mode '3').
If a planned order is firmed on header level only (PLAF-AUFFX) and has already been subjected to lead time scheduling, then it is scheduled again during the material requirements planning with planning mode 2 or 3, although the 'Lead time scheduling' parameter is not used. This is necessary because, due to the BOM explosion (planning mode 2 or 3), new component dates can arise.
Which planning mode is to be used in which situation?
Usually, it is sufficient to set planning mode '1' in the initial screen of the requirements planning transaction. If you actually need to process the material with a different planning mode, a respective planning file entry will be written, which is interpreted before material requirements planning. Planning mode '2' or '3' is then used automatically.
It is useful to set planning mode '2' or '3' when Customizing tables have been changed. No planning file entries are written when Customizing tables are being changed. There is also no where-used list showing which table entry is used in which materials. If, for example, you change the processing time for purchasing in plant Customizing of material requirements planning, this change is only taken over in existing order proposals if you run the material requirements planning with planning mode '3'.
Also refer same discussion on it,
MRP control parameters - planning mode
Best Regards,
R.Brahmankar