A Supporting Business Object is a Business Object that exists solely to complement, or support, a Major Object. Examples include Task, Journal, and Approval. Supporting Objects can be displayed on a Major Object's Form Arrangement, or as an embedded Form (ex: Specifics Form within an Incident). In order for Supporting Objects to appear as tabs or embedded Forms, they must have a Relationship with the Major Object they support (typically, the Major Object owns the Supporting Object).
Note: You can also display a Major Object as a tab on a Form Arrangement (ex: Problem might "support" Incident) or in a drop-down (such as Service) if you configure a Relationship (ex: Incident Links Problem). When you use a Major Object as a tab, it is read-only. For example, a Problem tab appears on the Incident record to quickly reference associated Problems, but the Problem records cannot be edited from the tab. To edit the Problem, click the GoTo button to navigate to the Problem record, or the Quick View button to open the record in a second window for a quick look at the details.
CSM provides several OOTB Supporting Business Objects (ex: Journal, Task, Approval, etc.). Use these as-is, edit them, or create your own using a Blueprint.