Is it compulsory (code required) to put a union between cold/hot connection and flexible hose?

[Curiousv]

This is branford white lowboy water heater and instruction manual says ..one should put union between cold water supply line and blue connector on top of tank ....
I am using connector like this - https://www.homedepot.com/p/1-in-FI...gated-Water-Heater-Connector-173218/306339160

Which steel flexible connector
My question is .. can I put this connector directly on to top of that blue connector ? or I must have a union ?

On hot side I did put a union because I had a union spare ..and flexible hose was little short but on cold side is it MUST?

What will happen if I don't put union and cold side between this flexible hose and blue connector?

my theory is ..less joints are less chances of leaks. @Terry

 

[Breplum]

Nut x Nut flex is equal to a union and best practice.
We've been putting flex on water heaters for 40 years, and a requirement in all seismic locales (all of CALIF.)

 

[Sponsor]

 

[Terry]

Flex connectors in Washington State.
That's Mt. Rainier in the distance. It's earthquake country.