A registered cellular device is a mobile device that uses cellular service, like a smartphone or iPad.

A registered cellular device (RCD) must be on your profile. It’s needed to use many of the features and services for digital banking. Those features include, but aren't limited to:

• Mobile app: Requires a RCD to access. 
• Money movement services: Zelle, internal and external transfers, etc.
• My profile: Address or phone number changes, manage username and password.
• DIY services: Manage account alerts, manage Shared Access users, lock or unlock your card. 

See also: What digital banking tools and features do you offer? and How do I establish a registered cellular device on my profile?