What does selecting an OS in VirtualBox actually do?

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http://superuser.com – In the setup process for a new virtual machine, VirtualBox presents several different OS types and versions to choose from: I'm curious as to what VirtualBox actually does when you select an OS. I'm sure, for example, that it wouldn't present the same hardware to a Windows 3.1 guest as it would to Windows 7. What I'm less clear on, however, are the potential differences between, say, a Linux and a BSD guest: if they're both designed to run on the same hardware, then what optimizations could VirtualBox possibly make if it knows which OS it's hosting? In other words, what types of change (HowTos)