The Delete button takes on dual meanings.  If the user is in the BOM Main Tab when the button is pushed, the button will delete ALL items on the BOM.   You will be prompted for your password. The following message will appear: 

If there are open work orders for the assembly, then the user is presented with a prompt advising that the bom cannot be deleted while open orders exist, as displayed below: 

If the user is in the Item Detail Information Tab when the button is pushed, the button will delete only the item highlighted.  You will be prompted for your password.  The following message will appear: