Set up any mappings for the variants in the feed. For example, you might want to change the apparel sizes to keep consistent filters on your site. The settings you apply here will apply to all feeds.
Often it's useful to change values for the products in your vendors store when publishing to yours. For example, your vendors may have a different sizing naming convention. Or, you might want to use a different product type.
Under the inventory feed settings, there is a mapping tab. Under here you can easily set the values you want to transform. For example, change a vendors "M" to a "Medium" in your store.