Just looked at a ticket with UA and AD. Had to call premier line and rep had to go through rate desk since originally priced out at full biz fare ($5k+) even though traveling in Y (should be around $1500). Rep desk said AD demanded ticket be in biz and then they took space away and could not book anyways. I am not sure what a codeshare exactly means...
(ok, I do; I am just venting for those folks that were going to respond)