Traffic selectors or proxy-id are part of the IPSEC VPN standards published for interoperability between vendors of site to site VPN devices. These are part of the communications that peers send each other to setup the VPN tunnel.
By default without any configured proxy-id or traffic selector SRX will send completely open proxy-id pair of 0.0.0.0/0 and 0.0.0.0/0 to that any traffic that is routed to the tunnel can use the connection. Routing then determines what hits the tunnel and your security policies what is permitted.
The use of traffic selectors or proxy-id is only needed when connecting to vendors that don't support using this default fully open proxy-id pair.