The custom attribute feature is the ability and option to add extra information about each device that is connected to your account.
For example, you are able to add a permanent phone number, country, or any other information that is unlikely to change and that is specific to a particular device.
To attach metadata (email, phone number, etc) to your devices, you must click on the edit button on your account's "Devices" page.
This will take you to the "Edit Device" page, and it is here that you can add the custom attributes, while using anything for the attribute names (e.g. "Email").
It should be noted that when you download your device list to Excel or CSV, the extra fields will show up there, too.
On the "Destinations" page, when you're editing or creating an Email or Word Destination, there is an option to deliver the form to an attribute you've added to a device. From the dropdown you can pick which attribute you would like to use.
Note: You'll need to have at least one attribute associated with one of your devices to see the dropdown when you're working with the Destinations.
As of yet, we have not deployed the integration of this feature into the other Destinations, but we will keep you updated on any changes.