I think what they mean is: Sonic speeds / Mach 1 brings a set of problems. Mach 2, 3, 4 don't add new ones. But Mach 5 does, hence warranting a different naming

I think what they mean is: Sonic speeds / Mach 1 brings a set of problems. Mach 2, 3, 4 don't add new ones. But Mach 5 does, hence warranting a different naming with hypersonic. At least that's how I understood the conversation.