I have seen various e-books on the Internet that have a (v5.0) in the name e.g.
Charles Darwin - On the Origin of Species (v5.0).epub
Initially I thought this had to with the version 5 of the DocBook, but I have seen other versions (v3.4) that do not have a corresponding DocBook version.
I have also seen this for e-books in .html and .txt files. Nothing in the those or the .epub file points at DocBook in any way.
What does this (v5.0) stand for?