BIG-IP : SNAT necessary if device is only gateway to internet ?

From my understanding about NAT/SNAT

NAT is one-to-one (just map real IP address and NAT address)

SNAT can be one-to-one or many-to-one (SNAT List) or many-to-many (SNAT pool)

If you want to access real server , You can access NAT address and it route you to real IP server

But You can't access real server from SNAT address (It's Secure NAT in full name though)

everyone Please fix me if I'm wrong :)