What is the difference between the "Allow Alternative Item" option in these two locations?

ERPNext User Forum
1

One is located in item and one is located in BOM, if I check it in one place and uncheck it in the other, what will be the effect of each?

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2

Please check this.

3

Sorry, I’ve read this document but still haven’t figured out the difference. I guess the option in item controls the switch in Stock Entry and the option in BOM controls the switch in Work Order.