In your opinion, what the definition of low-yield nukes for China, let say for DF-26 ? 10kt, 30kt or ???
It depends on what China wants to do with them.
American analysts... whether they be DOD alarmists or FAS low-key underestimating the arsenal... seem to think China purely desires low-yield warheads so as to have a way to proportionately respond to an American tactical nuclear strike.
If that is the case, it might make sense to have a variety of warheads or one with dialable yield. The lowest yield an American tactical nuclear warhead can be set to is as low as 300 tons, while the highest is 170 kilotons.
The current warhead yield on most Chinese warheads is 500 kilotons or so, which is very high for responding to a low yield nuclear strike. To give an idea of how small the yield of an American warhead can be, 300 ton airburst over Terminal 1 of Beijing Capital International Airport would not even shatter the glass of any building of Terminal 3.
If that kind of tiny yield is being delivered against a military target, or even a fully evacuated civilian target, it would not warrant completely leveling an American city or entire air base. So even very, very low yield warheads would be warranted for China.
If China actually desires to have some tactical nuclear capability... I have no idea why that would be needed, both CPC ideology and the raw strength of China's conventional forces favor nukes as a retaliatory weapon only... what yield would be best would depend on what role the PLA envisions for tactical nukes. But there is no indication China needs tactical nukes for military purposes, instead the only justification being to retaliate if any adversary uses low yield nukes on China first.
EDIT- To clarify, when I say "CPC ideology" I'm talking about the founding tenets that drove China's decision to build nukes and then maintain a minimum deterrent. It is possible this has begun to adjust given Xi's mission to build the PLA into a world-class military. If it has, I of course welcome corrections and information.