The encryption domain for each gateway is defined on the relevant gateway object.
For the pictured gateway:
If an interface has multiple subnets in the topology (because multiple networks behind it), we will summarize into the largest possible subnet.
The behavior depends on the setting of ike_use_largest_possible_subnets and your version as described here: New VPN features in R77.20
You can see all the SAs currently established on your gateway with the CLI command vpn tu.
What's the actual problem you're trying to solve here?