A matrix of rank k is equivalent to a matrix with 1 in the first k diagonal entries and 0 elsewhere.

Export will produce a single file with all the subgraph content. It will have *.html extension and will be viewable with/printable from your browser. The source text (except for a few html tags) will remain in markdown, just as you created it. You can convert it easily into format of your choosing, for details see Export from Knowen node.