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.