The XCT record specifies the beginning of an external cell cache and that specifies the beginning of a collection of CRN records as defined in the Globals Substream ABNF. The collection of CRN records specifies the values of cells in a sheet in the external cell cache.
ccrn (2 bytes): A signed integer of which the absolute value specifies the number of CRN records immediately following this record. A value less than zero specifies that the most recent SupBook preceding this record contains a link that is not valid.
itab (2 bytes): An unsigned integer that specifies a zero-based index into the rgst array of the most recent instance of a SupBook preceding this record in the file. The element of the rgst array specified by this field contains the name of the sheet that contains the cached cells. The value of itab MUST be less than the value of the ctab field of the most recent instance of SupBook.