The BOM attach workflow is a special case of the Part Reference workflow that has the same advantages as the X-attach workflow.
In this workflow, you draw the parts in the assembly drawing. The parts are also available in their own drawing files. In the assembly drawing, you place Part References on the parts. After that, instead of specifying the component properties in the Part Reference, you attach the BOM of the part drawing to it.
In the part drawing, you specify the component properties of the part in the Assembly Properties dialog box (using the AMPARTREFEDIT command). The Part Reference in the assembly drawing automatically retrieves the component properties from the part drawing, which in turn updates the BOM in the assembly drawing.
Like the X-attach workflow, the BOM attach workflow is not restricted to an assembly - part scenario. You can also attach the BOM of a subassembly drawing to a Part Reference in the assembly drawing. In the BOM of the assembly drawing, the subassembly is displayed as an expandable item. When you expand the subassembly, the parts of the subassembly are shown below it. You can build a hierarchy of assemblies and parts using this method.
Any change you make to the component properties of a part or subassembly automatically updates the BOM of the assembly drawing. If want to prevent the BOM from updating a particular component property automatically, you manually specify a value for it. All dialog boxes that display this component property highlights the overridden value in blue. If necessary, You can right-click a highlighted cell, remove the override, and restore the xreffed value.