The Advanced Scientific Data Format (ASDF) is used to store models for axes manipulation and rules that serve to exchange information with scientist of between stages of the calibration pipeline. This format is used by some reference files and also in some of the science calibrated products.
ASDF is a human-readable, hierarchical metadata structure, made up of basic dynamic data types such as strings, numbers, lists and mappings intended as an interchange format for delivering products from instruments to scientists or between scientists. For JWST, it is used to create reference files and share information about transformations using simple models and the rules to combine them. For more information refer to the JWST Science Calibration Pipeline documentation.