I suspect few people know what "none" means here, as they are most likely to pick one of the other options. None:  This could simply be the minimum partition size allowed by Windows (32GB). Or (less likely) it could simply be that Dell includes the OS media but no factory installation of the OS. 40GB Microsoft OS Partition Override:  Simple. Probably too little to be sustainable for MOST uses, but if you know what you are doing and need to streamline the C: partition, then this may not be such a big deal (the page file could be moved to another "disk" as well). I have a 2012 installation that has been sitting at around 35GB since December (including 8GB page file) - it is not a substantially utilized server, running as DC and WDS server (obviously the images are stored on a second "disk"). 80GB Microsoft OS Partition Override:  Again, simple. Would I deploy a server with an 80GB C: drive? No, I'd give it at least 100GB (depending on what it's for, I usually use around 120GB). However, 80GB - if you were stuck with it - would probably be manageable (if you could move extremely large page files to another disk). Maximum Partition Size Override for Microsoft Windows Factory Installed:  This will create a C: partition encompassing the entire size of the "disk".  If you have a 4x500GB RAID 10, with around 1TB of usable disk space, then your C: partition will be 1TB.  This will work for any "disk" configuration up to 2TB, which is why the GPT option is available. Maximum Microsoft OS Partition Override, GPT Enabled:  Say you have a 4x2TB RAID 10, giving you around 4TB of usable disk space. In order for Windows to see and use a "disk" that large, it has to first be converted to GPT. However, Windows can only boot to a GPT "disk" if UEFI is supported and enabled on the system. This would then give you a 4TB C: partition ... I'm not 100% sure that is how it would come, but according to their description, that would be my guess. Personally, I would select the 80GB partition. With 2008 and later, it is SIMPLE to extend and shrink partitions in Disk Management/diskpart, so I'd get it at 80GB, then extend to 120GB or whatever seemed appropriate for the server setup.  
It has been awhile since I've ordered a new PE, and I'm a little confused on this "OS Partitions" option. I am going with a 4 drive RAID 10 (500gb drives) and Windows Server 2012 Standard. Here are the available selections: I have called Dell and done the online chat and neither had a clue what these options meant. I have found some older threads talking about it, but none were really difinitive on how the server would come configured based on each option. The 40GB and 80GB are pretty straightforward, although even 80GB seems a little low these days especially with most servers having 16GB+ memory (swap), so it would be nice if you could choose a custom size. So how will the server come configured if I select "None"? And what about the two "Maximum" options?
"OS Partitions" option when building a new PowerEdge