Setting up an OpenVPN client to connect to the Synology diskstation VPN Server without redirecting all the traffic

I followed official Synology documentation and I created an OpenVPN Server to connect remotely to a NAS, that is in a LAN with its NAS_LOCAL_IP_ADDRESS.

Now, I’m trying to connect from my Mac via Tunnelblick following their instructions. In particular, they say:

Open the exported file openvpn.ovpn with TextEdit and replace YOUR_SERVER_IP with the public IP address of your Synology NAS. If you wish to send all traffic over VPN, remove # from #redirect-gateway def1.

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I do not want to reroute all the traffic to the VPN, I just wanna route requests to my NAS_LOCAL_IP_ADDRESS.

If I leave #redirect-gateway def1 commented, the VPN seems working this way. This is what I want but I cannot understand why it happens, since I’ve never specified the NAS_LOCAL_IP_ADDRESS anywhere in the config file.

Can you folks help me understand how things work under the hood?