ExFAT (Extended File Allocation Table) is a newer (2006) file system designed by Microsoft to be used on removable media. It is one of the file systems supported by TSK. ExFAT shares many properties with the FAT file system, but also has significant differences, the largest probably being that file data is no longer stored exclusively by FAT chains. See the exFAT Implementation Notes for details on how exFAT support was added to TSK.
Docs used during development: