I can't tell what any of it means... i see like 10 for dropbox on that list. he doesn't explain what shell icon overlays are and I can't tell what they are.
I think he's talking about the icons you see over folders in Explorer added by some applications. Dropbox will add icons indicating the sync status of your Dropbox folder over the folder icon itself.
Ahhh, it says a "few" and then three and then two... and I see 10 as well, and no overlays.
But, interesting, I never knew the overlays were limited.
When he says "Dropbox wins by having three spaces," he doesn't mean there are three entries. He means Dropbox prepends three spaces to the identifiers so they're sorted first in the list:
" DropboxExt01" vs. "DropboxExt01"
In Windows–land, the Shell is the GUI. It’s Explorer putting icons and things on the screen. Those icons usually represent files and directories. Each icon can be different, usually determined by the file extension or by metadata inside the file. On top of those icons you can add overlays. TortoiseSVN adds overlays to indicate the state of files inside of an SVN repository. Dropbox likewise adds icons to files inside of a synced folder. They have to fight because Windows is dumb.