[ ] means the argument is optional, but I've never seen it nested like this before. Anyway, I found this documentation to be a bit more clear:
cmd.load(string filename, string object-name, integer state,
string format, int finish, int discrete, int quiet,
int multiplex, int zoom, int partial)
It even shows how the function is implemented:
def load(filename, object='', state=0, format='', finish=1,
discrete=-1, quiet=1, multiplex=None, zoom=-1, partial=0):
filename is mandatory, the rest is optional with the above default values and types.