What's the use?
This feed is used to send more frequently price and availability updates, than a Google Shopping feed functioning through URL.
How to use this option?
It must be used in conjunction with a Google Shopping- Online Products feed functioning through URL.
Its setting is similar to Google Shopping channel.
This feed is working through CSV and has to be registered in your Merchant Center.
To know more about its use within Google, please consult their help center.
You are publishing your products on Google Shopping through API?
This option is already included in your catalogue, inventory update will be done more frequently using the API compared to a Google Shopping catalogue through URL. You don't need this Google Inventory Update catalogue.