On the function man page, ?mgoSim, the 'semData' argument is described as a GOSemSimDATA object. It looks like the maintainer didn't add an alias for the name of the class which would be found with ?GOSemSimDATA. Often class objects are documented with the '-class' extension so try ?`GOSemSimDATA-class`, note the back tics. On that page you'll find a minimal description of the object. If you still have questions I would suggest contacting the maintainer. You can find the contact info with help(package="GOSemSim") or on the package landing page: http://www.bioconductor.org/packages/3.5/bioc/html/GOSemSim.html.