JPEG XS is a compressed image and video format, and related standards. It is published as ISO/IEC 21122. As of 2019-08, all the critical parts of the standard had been published.
From the standard's table of contents, we surmise that there is a still image file format called JXS, and an ISOBMFF-based format called Motion JPEG XS. There is also a standard way to use JPEG XS in HEIF.
- ISO/IEC 21122 - JPEG XS low-latency lightweight image coding system. (These documents are probably not free to download.)