Pete Comment
Following a series of North Korean submerged, static pontoon tests of SLBMs (intended to be exclusively nuclear tipped) South Korea has just done a similar test. South Korea reportedly successfully tested a variant of the usually land launched Hyunmoo 2B missile with an estimated range of 500km.
All nuclear weapon states, which eventually developed SSBNs (from decades ago to more recently) went through a preliminary submerged pontoon test stage for their nuclear intended SLBMs. Their SLBMs were never intended to be merely tipped with a high explosive warhead. From pontoons the SLBMs graduated to the technically more difficult step of experimental SSBN or SSB test launches which can occur when the submarine is stationary or moving slowly (say 4 knots).
Significantly South Korea is the only non-nuclear weapon state to test a SLBM. The implication being South Korea feels it needs to make preparations to nuclear arm its SLBMs. This would make strategic and deterrent sense given the severe unstoppable nuclear threat South Korea experiences from North Korea. Basically the Dosan Ahn Changho-class submarines South Korea is gradually deploying would be uneconomical and irrational if used to merely launch high explosive SLBMs from their 6 silos. Uneconomical and irrational is if each of the 6 SLBMs have a one tonne high explosive warhead, equalling only 6 tonnes total throw-weight. This is less than the 7.7 tonnes of bombs/air-to-ground-missiles of a South Korean F-16 or more significantly 8 tonnes delivered by a South Korean F-35A (23 in service, so far).
SLBMs are only economical and act as a deterrent (their main purpose) if they are nuclear armed. Otherwise North Korea could feel comfortable first or second striking with its nuclear tipped SLBMs, land based missiles and free fall aircraft dropped nuclear bombs against Seoul and other critical targets in South Korea. And in any case the Law of Nuclear Ambiguity means that North Korea must assume that all South Korean SLBMs on their way to North Korea would be nuclear tipped and deserve a nuclear response.
For more details see NavalNews very useful July 4, 2021 report which adds:
"The first ship of the Dosan Ahn Chang Ho-class [aka KSS-3 (or no hyphen) or III (with or without a hyphen). South Korea desperately needs standardized weapons' designations!] is fitted with 6 cold launch VLS (vertical launch systems) reportedly capable of launching both the Hyunmoo 4-4 SLBM and a submarine launched land attack cruise missile (SLCM) known as Hyunmoo 3C with a range of 1,500 Km. The follow-on class, known as KSS III Batch 2, will be fitted with up to 10 of those VLS"[still only 10 tonnes total high explosive - less than 2 F-35As].