MPN values for each variation of your Configurable and Grouped Products can be provided in two ways – via Description Policy or the Manage Variations pop-up. If they are set in Description Policy, the relevant values will be taken from Magento Child Products. MPN values provided via the Manage Variations pop-up will have a priority over the settings specified in Description Policy.
For each variation of your Bundle, Downloadable and Simple with Custom Options Products, you can provide MPN values only via the Manage Variations pop-up.
The rest of Catalog Identifier values can be specified in eBay Global Settings.