Marking your listings as Multi-Item listings allows them to fully join the Reverb ecosystem, gaining them access to comparison shopping and targeted ads much more expansively than in the past.
Two key things are important to properly identify these listings:
1) Setting the multi_item field to TRUE on these listings in your inventory. You can do this by going to your Manage Inventory page, exporting your listings, marking TRUE under the multi-item listing column located near the right-hand side of the spreadsheet, saving the file as a CSV, and then uploading it in the Manage Inventory page.
2) Adding the UPC of the most expensive item in the group, which is considered the 'core item'.