The FBKFD structure contains information about a bookmark.
fbkf (4 bytes): An FBKF specifying further information about the bookmark.
cDepth (2 bytes): An integer value that specifies the number of bookmarks in the document of the same type as the bookmark associated with this FBKFD, the ranges of which overlap the beginning of the range of this bookmark. To increment the count, a bookmark MUST meet the following constraints:
The starting CP (cpS) and limit CP (cpL) of the bookmark, as defined in the specification of that PlcfBkfd and the PlcfBkld it is paired with, satisfy the following in relation to the CP (cpCur) that marks the beginning of the bookmark of this FBKFD.