Originally Posted by D582

ExpertFlyer displays inventory that AC has distributed via a GDS (a lot of EF data comes from Sabre) and would be dependent on the point-of-sale used (EF supports USA and UK points of sale). It will also be dependent on the entire itinerary (i.e. your profile says you are in Boston, so BOS-YYZ-TLV can show very different inventory on the YYZ-TLV segment than if you just searched for YYZ-TLV).

AC reservations would show the inventory they are willing to sell directly. It is certainly possible for them to not distribute all of their inventory via other channels.

Regarding seat selection, bookings made on AC's website will in general auto-assign a seat. However, bookings made via AC reservations will not. Neither will bookings made via a GDS, other third-party websites, codeshares etc.