The universal Color Exchange Format (CxF) was developed by X‑Rite and officially accepted as an ISO standard. Being an open format, it enables the precise communication of digital standards, measurement data, and metadata across different applications.
While a regular CxF3 file contains only information on solid colors, a custom CxF/X‑4 resource can also provide information on gradations. GMG OpenColor supports the import of such CxF/X‑4 resources and can thus also take into account existing data on spot color gradations.
You can find further information on these user-defined resources in part 4 of ISO 16972.
If you would like to know how the structure of a valid CxF3 file as well as a user-defined CxF/X‑4 resource should look like, you can find examples at https://standards.iso.org/iso/17972/-4/ed-2/en/.