Fast road riders who want aero on Giant's chart. TCR is the lighter, more conventional sibling.
Giant's aero race bike. Stiffer at the front, more wind-cheating than TCR, still in the same size-letter family so many riders can twin the two.
Gravel riders who want Giant's drop-bar all-road, not a TCR with knobbies.
Giant's gravel bike: stable, mounts, and a position between Defy and a race gravel whip. The X variants add more suspension/travel attitude.
Aero road riders on Cube.
Cube aero road. Deeper than Attain, race-leaning.