AC can reserve and confirm a seat on UA but it can't ASSIGN a specific seat on UA. I ticketed a YYZ-SFO flight on AC flightnumber but UA metal. The AC agent actually put me on hold while she contacted UA to have a seat assigned - she couldn't do that with the AC system.

On the way back from SIN, the agents in SIN could issue my SIN-NRT and NRT-SFO borading passes for UA, but not the connection on AC for SFO-YYZ. I had to get to the AC ticket counter in SFO to get the BP