A custom feed is a feed template that can be modified and is unique to your account.
Custom feeds allow you to control the fields, validation rules, settings and so on. This is opposed to common channel templates which are pre-built and maintained by our system and ready to be added to any store. If a channel updates their feed specs, then we will update the common feed template for all customers. Any modifications to the template fields of a custom feed will need to be maintained by you.
Cloning a feed creates a custom feed
When you clone a common feed we generate a custom feed using all of the existing setting you chose to clone over. This means you can customize a common feed template by cloning it. Cloning a feed allows you to add or remove feed fields, add or remove validation warnings and errors, change the file format, and so on.
Learn more here: How do I clone a feed?
Adding your custom feed template to other stores on your account
You can also make custom feeds available for any store on your account, across all of your stores. This is especially useful for agencies or anyone with multiple clients or stores. For example, when you want to customize your Google feeds or build a feed template for a special use case and use it for many of your clients.
Learn more here: How to make a custom feed available to all stores on an account