Are Tubes Safe In Radial Tires ?

There's several warnings, around, stating that it is unsafe to run tubes in radial tires, due to the sidewall flex of the tire. -- I see tubes advertised for use in, both, on-road, and off-road vehicles. These tubes are called "radial tubes." -- What does that mean ? -- Does it mean that they are safe in a radial tire - or - does it just imply that ? -- I do not plan to use a tube in a radial tire for on-road use. --- GR16 KR16 GR/KR-16 16" Tire Inner Tube for Auto, Truck, & Equipment TR13 Radial | eBay ------------------ I sent a message to this seller asking the same question re safety for on-road use. Awaiting answer.