No it does not have to be. But remember if it isn't the master interface then adat optical will not be an option for your sync settings. This is fine (or maybe even preferred) if you have a device such as a BRC that can accept and sync the ADATs to the word clock out of the ADAT Bridge I/O. Without one of these sync devices your adats will have to clock to the ADAT bridge I/O (hold set locate and press the digital in button on the original adats).