From my CCIE Lab preparation Note

outbound packets — change SOURCE    —inside source   <original ip><–>
outbound packets — change DESTIN      —outside source <–><original ip>

inbound packets   — change SOURCE    —outside source <original ip><–>
inbound packets   — change DESTIN       —inside source   <–><original ip>

*Packets that router generated will be NATed by single “ip nat outside” command in “destination NAT” form.

***If someone do not understand this, please analyze it by yourself. I almost forget everything about my notes. Maybe I will explain it later — may be 2 years later…….