A panel in STAAD.Pro Physical Modeler is a fictitious area which is used to distribute load to supporting members.
Notes on Panels
The panel feature does not work on complex or irregular areas. Panels should be symmetric and regular in shape.
Re-entrant (i.e., concave) corners should be avoided within panels.
For complex panels or framing, a clockwise or counterclockwise panel definition can sometimes change the resulting load distribution.