The "Pro" button negotiates the SPDIF and AES outputs between the consumer and professional standard language for status bit channel information, which is embedded in the digital word stream at the end of each block, or digital word.
Status bit channel info can tell the next device in-line information such as sample rate, bit depth, sync issues, known error corrections, known jitter specs, etc. The status bit information is conveyed whether the card is set to Pro or Con (card is set to Consumer when Pro button is not lit) but it merely changes the format to a Pro or Consumer device language.
Audio quality is not affected in any way, and in almost all cases a professional or semi-professional device will read both pro or consumer status bit language; but most consumer devices (a hi-fi stereo receiver or home theater product with digital input, for example) will only read the Consumer language format and you would not want the Pro button engaged in this rare application.
