View Single Post
It opens whatever kind of window you have configured it to open. See the New Windows section of the General preferences.

It opens the default OmniFocus document if you haven't got a window open for it for some reason (such as closing the last one, or following an omnifocus:// link from outside the program). Remember, you can open other OmniFocus documents (such as backups, the archive or exported copies of the database) by double-clicking on them in the Finder and have them open at the same time as the default document for doing comparisons, moving data from one to the other, etc.