What do the numbers after an invitee's status in iCal mean? After the text describing the status of an invitee in iCal (in the context menu for the invitee) there is a number. For example I nearly always see numbers like 1.1 or 1.2:



and occasionally I see "n/a":



More importantly, I've recently discovered that one of my invitees is consistently not getting my invitations, and his status (uniquely among all my contacts) shows a different number, 5.1:



What do these numbers mean and why does one of them seem to be associated with an inability to receive invitations from me?

I am using OS X 10.7.2, iCal 5.0.1, and iCloud. The invitations in question are all being sent to mac.com or me.com addresses.
These numbers are only present for users who have enabled the iCal "debug menu", using a third party configuration tweaking utility, or the command line

defaults write com.apple.iCal IncludeDebugMenu 1


which adds them, as part of an additional entry in each invitee's context menu, after his e-mail addresses:



The numbers themselves contain information about the invitee's version of 
MobileMe or iCloud.