The ADAT format allows 8 channels of digital audio on a single optical connector, but only at sample rates up to 48kHz.
At 88.2kHz and 96kHz, the S/MUX format must be used instead. S/MUX is a
way of squeezing higher frequency signals into an ADAT carrier. Twice as
many samples are transmitted for each channel, so there can only be
half as many channels.