BIG-IP : how to determine Self-IP used by SNAT ?

Auto map is one form of SNAT. Secure (or source) network address translation is the method used to translate the client source address in a request to an address controlled by the proxy, usually for the purpose of controlling return routing. Auto map just uses the self-IP of the LTM as that source address, while a SNAT pool allows you to define specific non-self-IP addresses. Since a single IP address can only support about 64,000 simultaneous connections (based on available ports), a SNAT pool is often used when you might need more than that.

If you have multiple self-IP addresses on the LTM, for different VLANs, the device will use the one that makes the most sense for routing.