It is a relatively new road chain width standard. 11/128 means a width of 11/128 inch as popular single-speed widths are 3/32 and 1/8. Initially, it is invention for road bikes with multy-speed cassettes and name "KMC X11" states for 11-speed chain. Chain rollers have 2.18-mm nominal width so cassette sprockets and chainrings must be of 2.0-mm width.
German women reportedly used these chains and rings at Berlin 2020. Dutch machos clearly stayed on 1/8 Izumi V-Chain.