Quick tissue check. When tissue is removed at surgery, the pathologist might need to look at it under the microscope if cancer is suspected. Since the tissue needs to be sliced thinly to look at it, the quick way is to freeze the tissue, then slice the fronzen specimen. Hence frozen section. Most of the tissue is imbedded in parafin, and processed later, the permanent section. The permanents are more accurate.
Intraoperative test. If a lesion is seen during surgery, part of the lesion can be examined by a pathologist. The tissue is frozen and cut into thin sections that are examined microscopically to provide a provisional diagnosis. It can be used to diagnose cancer, but results are considered preliminary. In practice if the pathologist diagnosis cancer on frozen section, surgeon with treat it as such.