If you setup Biditem Parent-Child relationships you should try the “Quick Parent-Child Relationship Setup”. It brings up a dialog box where you can simply drag and drop child items into Parent Biditems without even setting the biditem as a Parent (dragging it to a biditem makes that biditem a parent). Left and right arrow keys will also move items in and out of Parent biditems. Sometimes faster to use this feature than setup an Excel spreadsheet with the relationships.  Users tell me they love this feature.  Remember that Parent biditems cannot have activities. The example I have shown below is a parent biditem for RCP pipe and then the child biditems that are the different sizes of RCP.

Quick Parent Child