PixInsight introduced a file structure called Extensible Image Serialization Format or XISF. This format was written to replace the FITS format, commonly used in astrophotography. However, this format was essentially designed by, and for, the PixInsight software, and many important types of metadata are stored in private namespaces that the Pixinsight developers advise cannot be relied on and may change arbitrarily between XISF specification versions. Moreover, the way a keyword is used to describe a given feature can be very opaque. As a result, the advantages it could bring for use within Pixinsight are generally nil for other software. Moreover, the FITS file format is a recognized and widely used format in the community. NASA itself uses it for its own images. It's very flexible and powerful, and contains all the features needed for modern astrophotographic processing.
We have therefore decided to enable Siril to read this type of file, but in read-only mode. Saving in XISF format is not, and will not be, implemented.