Add a field to your Data Model either of the Pega out-of-the-box Data Type, or ideally a customer-specific subclass of it. 

By adding the Data Object you need to the Application, it will appear in App Studio's Data Objects listing, from where you can maintain the list of Fields and encapsulate Views that become available to any Case Type composed of that Data Object.

Discussion on this topic was sought from the LSA Data Excellence (Pega 8.4) webinar conducted in July 2020. The webinar and its full set of discussions that arose from it are available at LSA Data Excellence: Webinar, Questions & Answers.