Single Action- The project will do any next action handling.   This is for a project when the tasks are not related to each other.  The project is made up of "one off" tasks.

Parallel- The project can have multiple tasks marked as next action working in "parallel".

Sequential- The project can have one next action task and when that task is completed another task in the project can be designated as the next action  result in a "sequential" handling of the next actions.