pm2 -i flag runs app instances in fork mode rather than cluster mode

I am using pm2 version 5.3.0

I want to run several instances of my application in cluster mode using CLI.

I use the command

pm2 start node -n my-app -i 2 -- build

But when I provide -i flag, it starts a number of instances in fork mode, not in cluster mode.

In their documentation they say using -i flag with start runs the app in cluster mode. This contradicts another part of the documentation where they mention adding a config file as they say by default pm2 will start the processes in fork mode (which what happens) and I need to set exec_mode field to run the app in cluster mode.

So, is there a way to use the pm2 start command from cli AND specify the execution mode as cluster mode? I can’t seem to find any flag to set that.