One more thing, The proxy ID are used both in route-based and policy-based VPNs.
The proxy ID generation for policy-based VPNs is based on the security policy bound to the VPN, and it cannot be overwritten with the proxy-identity command under the 'set security ipsec vpn <vpn> ike proxy-identity' stanza. The proxy-identity is based upon the source-address, destination-address, and the application listed in the security policy.
A traffic selector (also known as a proxy ID in IKEv1) is an agreement between IKE peers to permit traffic through a tunnel if the traffic matches a specified pair of local and remote addresses. With this feature, you can define a traffic selector within a specific route-based VPN, which can result in multiple Phase 2 IPsec security associations (SAs). Only traffic that conforms to a traffic selector is permitted through an SA.
reference:
https://www.juniper.net/documentation/en_US/junos/topics/concept/ipsec-vpn-traffic-selector-understanding.html
https://kb.juniper.net/InfoCenter/index?page=content&id=KB29364&actp=METADATA