Are people confused about the difference between China's national security law and the new Hong Kong national security law enough that I should write it up?
The 2015 National Security Law is less a law for direct implementation than a policy declaration.

It sets forth an overarching national security perspective that guides and frames almost every aspect of China's legal system. https://www.chinalawtranslate.com/en/2015nsl/ 
The 2015 NSL defines national security very broadly, not just as combatting foreign threats, but as addressing potential instability from any international or domestic barrier to political stability, economic development, or the people's welfare.
This expansive notion of security and stability expressly includes food safety, cultural integrity, capacity for innovation, cybersecurity, environmental protection, energy self-sufficiency, financial risk management, defense capacity, and so forth.
A lot of the content on fighting hostile foreign forces and so forth was once in the national security law, but was shifted to the 'counter-espionage' law. https://www.chinalawtranslate.com/anti-espionage/ 
By Contrast, the HK National Security Law (PRC Law on the Preservation of National Security in the Hong Kong S.A.R.) is a very specific and designed to make it easier for China's central government to act more directly in HK. https://www.chinalawtranslate.com/bilingual-hong-kong-national-security-law/
It does this both by creating mainland institutions in HK, and taking greater control over existing HK institutions.
Unlike the mainland NSL's overarching framework calling for further legislation, this law directly creates new structures and enforcement mechanisms for HK.
I wrote about the impact on criminal procedure, explaining that cases of Subversion of State Power, Terrorism, or Collusion with Foreign or Taiwan Forces are now pursued and prosecuted under a two-tier system, which I labeled 'bad and worse.' https://www.chinalawtranslate.com/insecurity-law/ 
In the bad situation, special HK police teams enforce the law under the central government's oversight.

In the 'worse' scenario, the central government takes a more direct hand in investigating the case, and even applies mainland law to govern the investigation and trial.
The HK National Security Law is a HK Law, and generally aimed at HK- focussing on threats to China's national security that arise in HK.

It grants new criminal powers and creates new bodies, to ensure HK doesn't threaten China's broad understanding of national security.
Now, back to the Mainland where the NSL is certainly not the only law about national security.

There's a whole Chapter of crimes called 'Crimes endangering national security, and these include separatism, subversion of state power, insurrection etc. https://www.chinalawtranslate.com/en/criminal-law-2017-revision/#_Toc439590530
And China's Criminal Procedure has all kinds of special provisions for national security cases.

Access to a lawyer requires permission, witness identities can be concealed, 'RSDL' detention outside a prison may be applied, notice to families may be waived, etc.
This is the criminal procedure that may now be applied under the HK NSL 'worse' track.
Nat. Security offenses in the mainland were once called 'counter-revolutionary' crimes, and while criminal procedure has improved a lot that might help frame things.
You are seen as the enemy.
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