[whpalmer4]

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.