the instructor said that Virtual server and SNATs process traffic in one direction only where as NAT can process traffic in both direction.i understand what he means is virtual server and snat create one listener oject (i.e. virtual server creates destination listener object. snat creates source listener object) but nat creates both destination and source listener objects.
The BIG-IP system creates a source and destination listener for local traffic objects such as NATs that listen for requests matching a destination host IP address, or requests originating from a host IP address defined on the BIG-IP system.sol9038: The order of precedence for local traffic object listeners
http://support.f5.com/kb/en-us/solutions/public/9000/000/sol9038.html
for example, when creating nat address 1.1.1.1 for origin address 2.2.2.2 (i.e. nat 2.2.2.2 to 1.1.1.1), external client can initiate connection to 2.2.2.2 through 1.1.1.1. also, 2.2.2.2 can initiate connection to external resource and external resource will see it is coming from 1.1.1.1.
anyway, if you create virtual server 1.1.1.1 with pool member 2.2.2.2, external client can initiate connection to 2.2.2.2 through 1.1.1.1 but 2.2.2.2 cannot initate connection to external resource unless you configure another wildcard virtual server to handle outbound traffic from pool member.