The argument should be given in terms the source understands (e.g.a revision number for Subversion sources, or a hash for git sources).To erase named branch markers, convert it to "default" with a branchmap like Normally convert will import revisions in an order that produces the fewest jumps between branches in the commit log.
Its main use is to put a (converted) history at the end of another one by specifying the last revision of the existing history as being the parent of the new one.
Incremental conversions may be useful not only when tracking newer changes in the source repo but also in very huge repos which would need huge resources for a whole conversion and can be better handled with an incremental one. The argument should be a simple text file that maps each source commit author to a destination commit author.
It is handy for source SCMs that use UNIX logins to identify authors (e.g., CVS).
It can also be used to filter Mercurial repositories to get subsets of an existing one. This file is automatically created and updated on each commit copied, its purpose is to track which commits were already imported and which were not - and thanks to it allow to resume interrupted import and to make incremental updates.
It is important to note that this option is provided.