This option tells makeskel to create an update file: it will read description files (section 3.2.8, page 42) and will only create documentation nodes for identifiers which do not yet have a documentation node in the read documentation files. The output file in this case can be merged with one (or more) of the documentation files: it’s name should not appear in the list of existing documentation files. The makeskel program will do some elementary checks on this.