TZ 300 and TZ 400 are Gen 6 SonicWall Appliances




You can build an Active / Passive cluster noted here:

https://support.software.dell.com/kb/sw11944

So the answer of HA availability stands true for the TZ 300 and 400

However, Stateful HA (Retaining a secure web session) which is Optional is only available on TZ500 and up noted here:

https://support.software.dell.com/kb/sw11944