When using a 2×6 i/o module in a Symphony I/O (MkI or MkII), the 2×6 i/o module shows up as a 16ch Avid HD I/O in Pro Tools HD. Here is how the I/O corresponds with the ProTools I/O (Note that PT inputs 3 through 6 are nothing. This is so all the input and output channel numbers line up with each other when using hardware inserts):

 

Pro Tools Inputs 1-2             Line Inputs 1-2
Pro Tools Inputs 3-4             N.A.
Pro Tools Inputs 5-6             N.A.
Pro Tools Inputs 7-8             AES Inputs 1-2
Pro Tools Inputs 9-16           Optical Inputs 1-8

Pro Tools Outputs 1-6          Line Outputs 1-6
Pro Tools Outputs 7-8          AES Outputs 1-2
Pro Tools Outputs 9-16        Optical Outputs 1-8