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 racers coming from a road-race fit who found Checkpoint too tall or too touring-oriented.
The gravel-race Trek: lower, longer, and more aero than Checkpoint. It wants speed on dirt, not a full frame-bag tour.
Aero racers. 0 SLR / Verticale if you want light.
Wilier aero race bike. Deep, fast, and recently updated (ID2) in a way that blurs 'who's an aero bike for'. Still a race stack/reach chart.