The only time I've ever been confused (in wedding invite terms) was by an invite to the church ceremony, which took place *after* the official wedding. I think it was some kind of catholic blessing, but they couldn't have (or didn't want) a catholic wedding as they already had a child together. So it was like a wedding, but with no actual getting-married part, as all the legal stuff had been done in the registry that morning.

*coughs sheepishly* and yes, I didn't actually R to the RSVP did I? I'm sure you knew we were coming but I still should've replied! Sorreeee.... :-)