The purpose is to have the sales order configurator ask how many of each item to include. This is done as an item is entered on the BOM Child screen: the part number is specified, the type is set to Base Item, and the Adjustable option is checked. This will add the small white plus sign (in red, as circled) to the bolt icon on the BOM. Lastly, enter the question that the configuration is to ask in the Adjustment Description field. In this case, all the items can have their quantities specified..
The use of Adjustable Raw Material permits one inventory item number to be used for a Kit, or any BOM, where the child parts remain the same but one or more of them can have different quantities.