A Next Action is a label that you can apply to your tasks inside of a project.  When tapping the Action field a list of options will be displayed.  You can choose the appropriate label from the list to apply to your task.

Choosing the label "Next Action" will move the task with that designation to the top of the task list inside of any project type.

Single Action- This project type 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- This project type can have multiple tasks marked as Next Action working in "parallel".

Sequential (GTD)- This project type 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.

If your project was set to use Parallel or Single Action you can designate multiple tasks as the Next Action and all will move to the top of the list inside of the project.

If your project is set to Sequential (GTD) then you can only have one task designated as a next action at a time.