@tueng a simple workaround to your solution is maintain the same BOM structure, however if A1 is backordered, you simply remove A1 from the list of components in the Work Order and replace it by either A2 or A3. Another option is if you already are using Advanced BOM feature in your account, create 3 item revisions with each revision having A1, A2 or A3 as the differentiating component, Then you can simply choose which item revision to use in the work order. However, I wouldn't recommend enabling the Advanced BOM feature just to cater to your requirements.. It's better you follow option 1. It would just entail too much work.

My concern is: Opt 1: Have no idea user can remember to replace the correct component and the quantity also --> which we want to minimize the faulty input. Opt 2: I tried but possible if create new BOM not new revision because of effective date. One more question: what if I create BOM with Matrix component ( Mother code, not child code)? (sorry because I am new to NS and I tried on our demo system but I can not create matrix Items correctly to create BOM)