How is a part that has no visible edges labeled?

When you have a part in the middle (e.g., stiffener without chamfers) that you would like to label on a side view (in the figure on the right):

Before creating the cut view, you must activate the "Enable annotations on elements invisible in details" default (in Management Tools - Defaults):

The cut view contains a point in the part that has no visible edges (see inside the cloud in the figure below):

Label the part by selecting the point during the "Insert label" command:

A label for a part without visible edges is created:

Important: The view, where you want to label the invisible part, must have been created with a presentation (drawing style) where visible and hidden lines are switched on (see figure below):