You may be asked to demonstrate breadth through a qualifying exam. The RPE requires a written paper and presentation, which gives faculty the opportunity to assess the potential of the student to conduct independent research. Conducting research for your Ph.D. is much different from taking classes. It's like hiking a well-established trail up a mountain. Research lacks the familiar, comfortable milestones of course work.
Research is free-form, the expectations less concrete.
It will be up to you, with the help of your advisor, to set your own checkpoints to evaluate your progress across the plains of research. Send e-mail and tell us.