Using the cl_context_properties parameter

Can anyone please tell me how to specify the “cl_context_properties *properties” parameter found in the “clCreateContext” and “clCreateContextFromType” functions?

I see that neither the OpenCL- specification examples (D.1 and D.3) nor the Nvidia SDK examples make use of this parameter (it is always set to NULL).

Is this parameter supposed to be used and, if so, in which cases is it supposed to be used?

Thank you for your help.