Many of Tuesday's events regarding North Korea can be filed under "we've been here before": the early indications of a seismic event the revelation that North Korea had given prior warning overnight to the US and China the steady stream of international condemnation the triumphant statement from Pyongyang. But within that statement were a couple of lines that indicate North Korea might have made a significant breakthrough.
First was Pyongyang's assertion that the nuclear test was "carried out at a high level in a safe and perfect manner using a miniaturised and lighter nuclear device with greater explosive force than previously". In short that it had achieved a long-sought goal, making the bomb smaller, perhaps small enough to place on top of a missile.
Three things to note: it's something the US and its allies have been expecting to happen, eventually North Korea doesn't yet have the missile capability to send a warhead into space and back to a target on Earth and it knows a first strike would be a suicidal move, inviting regime-ending retaliation. But if true, it marks a real advance in North Korea's nuclear capacity, and leaves other regional powers with a growing nuclear threat to contain.
The other phrase that sticks out is North Korea's claim that the test "demonstrated the good performance of the DPRK's nuclear deterrence that has become diversified". As well as the potential for miniaturisation, much pre-test speculation had centred on whether it would use highly enriched uranium, rather than plutonium, thereby "diversifying" the nuclear arsenal. Uranium enrichment facilities are much easier to conceal than plutonium-producing reactors, making the threat of proliferation harder to counter.
South Korea says it might take 2-3 days to confirm what kind of fissile material was used. It's not always possible even to do that, let alone to ascertain the full details of a test deep inside a remote group of mountains. For now, what Pyongyang says happened is all there is to go on. But that's more than enough to attract another round of sanctions at the UN.