Software design engineering may include:

PLC ladder logic software.
Allen-Bradley SLC-500/Micrologix/Contrologix, Automation Direct,
Toshiba, GE, and others.

MMI (Man Machine Interface) software.
Wonderware, Allen-Bradley Panelview, TCP touchsceens, Visual Basic,
html.

Software documentation, making sure the customer always has complete up-to-date source files.