EPub is the file format used for eBooks and is readable on just about every system in use today. Basically everything except for Amazon’s Kindles. The advantages of the ePub file format is the ability to distribute a cross platform document that is readable on different screen sizes. The ePub will re-format or reflow the text so the document is viewable on small displays such as phones or large displays such as desktop monitors.
Outline mode in Google Docs is automatically generated, either from styles applied to the document through the header styles or programmatically, whereas Google Docs tries to figure out where the logical breaks in your document belong. This feature shows up in Google Docs on the web and on Android. I could see it being very useful to navigate long documents on handheld devices such as mobile phones.