Originally Posted by MSPeconomist

The U is apparently because the RJs don't have FC. Try purchasing it as two separate tickets. Alternatively get rid of the A segment as delta.dumb doesn't like to combine these with RJs not having FC on the same ticket.

For example, try MSP-DTW-XXX, where XXX iis CRJ-200 only. It's easy to find A class on the MSP-DTW connection but you apparently cannot book this as a connection to the CRJ-200 even if you break the fare. This was what I observed several months ago.