I want to learn more about Ebup 3 implementations. Epub files can be reflow is what they say but what is the exact meaning of reflow is not clear to me? Are there any standard specifications of document reflow ?
A "reflowable" book is one which doesn't use fixed layout pages: so page numbers can change depending on the font, font size, margins, and device a reader is on. For example: a reflow book on a large tablet will have fewer pages than the same book on a small phone: and the page numbers will obviously be different.
There are some great examples and tutorials for learning about ePub3 (the latest ePub spec) here: http://www.ibm.com/developerworks/web/library/x-richlayoutepub/index.html. There are absolutely specifications (set out by IDPF) for the ePub spec, the link above shows many of them...
Best of luck!