This configures whether B-channel selection for ISDN calls is dictated exclusively by the central office switch or merely preferred and negotiable by this switch itself. Set to exclusive(1), the card must honor whatever channel the CO assigns even if it conflicts with the card's own channel allocation plans, while preferred(2) lets this switch push back and pick a different channel; the wrong setting shows up as calls being rejected or routed to unexpected channels when the two ends disagree on who's in charge of channel selection. For example, interoperability issues on a PRI trunk get resolved once the admin discovers cardISDNChanId was set to preferred(2) while the carrier's switch strictly enforces exclusive channel assignment.