A "bit perfect" audio capture means that Every. Single. Bit. in the capture is the same as what's stored on the digital medium (in this case, the digital track on an LD).

Some older sound cards did digital post-processing (programmed in the driver) on the TOSLINK input (e.g., band-pass filtering) without notifying the user and without giving the user a way to disable it. Hell, some even did it in the hardware.

I know this sounds absurd since all the sound card needs to do is dump the PCM bytes it receives over the TOSLINK input onto the HDD, but this wasn't (and isn't) always the case. This is why some sound cards advertise "bit perfect" audio capturing.

It's not a gimmick. It's a response to the stupidly-written overzealous drivers of yesteryear.