DAQmxSetCIEncoderAInputTerm arguments

I am trying to translate this .NET command to it's c equivalent:

myCounterTask->CIChannels[0]->EncoderAInputTerminal = "/Dev1/PFI0";

The c function would be:

DAQmxSetCIEncoderAInputTerm(myCounterTaskHandle, "/Dev1/PFI0", const char *data);

Except I have no idea what "data" refers to.  The .NET equivalent has no such argument and there is no obvious data associated with this operation beyond the channel name.  I tried looking it up in the NI documentation, but it is totally useless (just a copy and paste from the header file with no actual information):

http://zone.ni.com/reference/en-XX/help/370471AA-01/mxcprop/func219d/

Anyone used this function before and know what the last argument should be?