For a Participant, Viewer is the Screen Sharing area that appears in the Participant console for Screen Sharing.
For the Leader, Viewer is the area within the dashed lines (Desktop Sharing or Region Sharing) or the application window (Application Sharing).
The status bar (bottom of the screen) will display:
the participant sharing
the participant with remote control
a list of participants currently using whiteboard tools (each participant name will be displayed in the current color he/she is using) if one or more participants are using whiteboard tools
the active speaker(s).