The Large Address Aware in the header file of the .exe. It allows an application to access more than 2gigs of memory at once.
Being that Skyrim actually has pretty forgiving requirements, those of us who had plenty of ram but maybe not screaming fast GPUs could use a tool to modify the Skyrim.exe to LAA. Made it run better and crash less for me. For those with kick ass machines, it allowed them to use high-res textures made by modders, 2000ish size textures instead of 1000ish. LAA made that possible.
Everything was hunkey dorey for a week. But even before fixing their own broken UI, they released a patch that upped the DRM specification of the game and made the LAA impossible. Steam had a problem with it and so did Bethesda apparently.
And rather than doing the sane thing, like making it a configuration option, or making it part of the configuration tool when Skyrim detects your settings.....they did the predictable. They patched the .exe and Steam locked it down so if you change the .exe to be LAA....Steam won't launch the game.
Reasons are myriad but it's my guess that Skyrim being so popular, combined with a ton of people using an .exe hack (for completely legitimate reasons) made Steam shit a brick. Because hey, how do they know if those modified .exes are just using LAA or are in fact modified to avoid Steam DRM? Last I checked on my favorite torrent site, there were 15,000 seeds of Skyrim.
So yeah. Pardon my lack exposition. It's just after discussing it in 12 places on the internet, I didn't come here to discuss much. I came here to say fuck and vent