The BKF structure contains information about a bookmark.
ibkl (4 bytes): An unsigned integer that specifies a zero-based index into the PlcBkl or PlcBkld that is paired with the PlcBkf or PlcBkfd containing this BKF. The entry found at that index specifies the location of the end of the bookmark that is associated with this BKF. Ibkl MUST be unique for all BKFs in a given PlcBkf or PlcBkfd.
bkc (2 bytes): A BKC that specifies further information about the bookmark.