Just running Leo again after a while, noticed "File > New" creates a tab "Master: untitled". This is different, so what is meant by "Master: " - Not the branch surely, (although I am indeed running it from my local git repository with the Master branch checkout). Perhaps Master in the sense of 'master-template', though their seems no special structure to the "untitled" document.
If the later is the case we could use to support things like making example documents that when saved become "Working Copies" and don't modify the "master" they were open from.
Tom