The Offset field contains the offset in the file at which the bookmark is set. It is initially specified in hex, but one may be entered in decimal by checking the radio button.
The Struct Name field contains the name of a structure with which the bookmark is associated. This is blank if the bookmark is not supposed to be associated with any structure. The button next to it brings up the structure selection dialog to pick a structure from.
The Line Length field allows a row length to be associated with the bookmark in hex or decimal. If this field is blank or contains no number, then no particular row length is associated with the bookmark. A length of 0 is not allowed, and currently no row can be longer than 1024 bytes (not that I can figure out why anyone would want one that long anyway).