The reason is due to I/O labels. When mirroring is turned off, the I/O labels for inputs/outputs/sends/inserts change. The list actually grows and shrinks depending on mirroring status and optical input format status. In order for us to change I/O labels, a session cannot be opened. This explains why Pro Tools generates a dialog box if you try to change Playback Engine. If you have mirroring on, the S/PDIF ports mirror analog outputs 1-2. If you turn mirroring off, S/PDIF ports show up under inputs/outputs 17-18. Also, if you change the optical format to S/PDIF, your 8 channels of ADAT optical will shrink to 2 channels of optical S/PDIF.