[whpalmer4]

Window title bar with no diamond: no changes of any sort
Window title bar with filled diamond: content changes, not yet saved
Window title bar with clear diamond: no content changes, but view changes, not yet saved

An few examples of the last would be hoisting a different section of the outline, scrolling elsewhere in the document, collapsing or expanding some rows. You can save the file and preserve this information, but it won't pester you about doing so if you quit, unlike if you have made actual content changes.

I wondered about this for a while myself, and just happened to stumble across the answer while looking for something else :-)