How is SSH_AUTH_SOCK setup and used by ssh-agent?

Question

I have been able to setup the sharing of ssh-agent for public-key authentication after reading https://superuser.com/a/230872/301446

The environment file thus generated

Answer 1

From here:

$SSH_AUTH_SOCK contains the path of the unix file socket that the agent uses for communication with other processes. This is essential for ssh-add.