You want to enroll an SRX Series device with Juniper ATP Appliance.

This is the port used for encrypted communication between the SRX series device and the Juniper ATP Appliance

In order to enroll an SRX Series device with Juniper ATP Appliance, the firewall device must have port 443 open. Port 443 is the default port used for HTTPS traffic, the communication between the SRX Series device and the ATP Appliance needs to be encrypted, that's why this port should be opened.