Generally refers to the telco switch protocol used on an ISDN line, e.g. NI-1 or DMS-100.
The brand of equipment and software revision level that the telephone company uses to provide you with ISDN service. There are only a few types of switches in the world and usually just one in countries other than the United States.
The type of ISDN network you are connected to. This information is available from the ISDN provider and provided to the buyer when purchasing an ISDN line.