In my example I set the env var on the same line as the command, so the env var is only set in the new process, per open group:

> If the command name is not a special built-in utility or function, the variable assignments shall be exported for the execution environment of the command and shall not affect the current execution environment

Eg

FOO=bar myapp

FOO=xyz myapp

echo $FOO # prints empty string/newline