Okay maybe I can explain what happens with VirtualBox on OSX.
1. If VirtualBox is open you can use Add Machine and select the .vbox file to add it.
2. If VirtualBox is not open dbl-clicking on a .vbox file that isn't registered will start VirtualBox and add the guest.
3. If VirtualBox is not open and the guest is already registered, dbl-clicking on the .vbox file will start the guest.
As far as the alias you create by right clicking on the guests name and selecting "Create Alias on Decktop". That creates an alias that points to the .vbox file, which in turn launches the guest. The reason I was having an issue as Chip mentioned, was somewhere along the line, months ago, the file association for .vbox files was changed on one of my Macs so that if you dbl-clicked on a .vbox file or its alias, it would open TextEdit instead of vmstarter which is what it should be. So TextEdit would just open the .vbox file so you could read it.
The app Chip linked for me made it possible for me to correct the file association.
And like Chip, I haven't found a way to inspect the alias file itself.
Just to add, assuming the file association is correct, if you dbl-click a guest's alias it will launch the guest. If you drag that same alias to OSX's dock, you click it once to launch the guest.
OSX, Linux and Windows Hosts & Guests
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