Window Intercom System Expansion Set The NF-CS1 Expansion Set is designed exclusively for use with the NF-2S Windows Intercom System and consists of a System Expansion Sub-Unit and a Distributor for sound distribution. The NF-CS1 is designed for use in conjuction with the NF-2S, allowing the expansion of the coverage area for assisted conversations, even in compact meeting spaces with limited numbers of people and partitions installed.