Human Interactions is an element in the BPEL4People namespace that is an extension to BPEL. This element contains the child elements you can use to build a People activity, namely Logical People Groups, tasks and notifications.
You can add a Human Interactions element to the process or to a scope. As you define Logical People Groups, tasks and notifications at the process or scope level, they become available to the People activities at that level. Using a Human Interaction is a flexible and optional choice. The benefit is that the people assignments, tasks, and notifications within it are available to all People activities in a process.