Panel design could mean two closely related things. One could be the design of the relationships and connections between the electrical and control system in a panel (drawing the schematics). The other thing (which is closely related) is laying out the physical location of those components within the panel. Usually, you have to do both if you are designing a control system.
For panel layout, you need to physically arrange the components (usually using CAD software) to fit everything into the panel while taking into account efficient use of space, heat dissipation, compliance with electrical codes and regulations, easy of assembly and repair. In addition, you need to take into consideration how the finished panel will fit into wherever it is being installed.