When you add a new frame to the document using the Create Frame tool, it will be of type Unassigned. Unassigned frames are neither text frames nor image frames.
The Unassigned-Frame menu allows you to choose what type of content the frame will have (i.e. Text or Graphic), as well as insert that content.
NOTE: This section gives a high-level description. For more in-depth explanations, please refer to the appropriate parts of the Knowledge Base.