If it is a regional airline flight Ill put in the marekting carrier's code down. So for an Atlantic Southeast Airlines flight I'll record it as Delta Air Lines DL4734 and then in the comment field put in Operated by Atlantic Southeast Airlines as Delta Connection.

If the codeshare flight is operated by another carrier that sells its own tickets I would record that actual airline. For a Delta codeshare say on Northwest, I would record it as Northwest Airlines NW1610 and then as the comment put Codeshare Flight Ticketed as Delta 6813.

Besides this info, i'll put if I was upgraded or if I was an award ticket. Does anyone else use the comment fields for data like this, or just leave it alone?