Checked = Patterned with alternating squares of color.
Luggage= cases used to carry belongings when traveling.
Therefore "checked luggage" is cases patterned with alternating squares of color used to carry belongings when traveling.

OOOPS...I guess "checked" could also= consign for shipment on a vehicle. In that case "checked luggage" = cases used to carry belongings when traveling, consigned for shipment on a vehicle.
I am sorry to have confused you SPIFF, I imagine you were befuddled by the idea that 100ml bottles can only travel in cases that are patterned with alternating squares of color.
Should I have said checked baggage? Or something else? How about "Put your 100ml bottles is the bags that you will be checking at the ticket counter to travel in the belly of the airplane."?