XSA: HDI Containers real world examples

I've been following Thomas Jung and his posts on the new developer features. We are just moving into the XSA from XS Classic and am trying to get my head around HDI containers. Are they created per project or per application component (like SD, HR etc)? Can anybody give me real world examples of HDI Containers? Ofcourse these are accessible only through the WebIDE...is that correct?