ICMP flood
Use the ICMP flood IDS option to protect against ICMP flood attacks. An ICMP flood attack typically occurs when ICMP echo requests use all resources in responding, such that valid network traffic can no longer be processed.
The threshold value defines the number of ICMP packets per second (pps) allowed to be send to the same destination address before the device rejects further ICMP packets.
IP spoofing
Use the IP address spoofing IDS option to prevent spoofing attacks. IP spoofing occurs when an invalid source address is inserted in the packet header to make the packet appear to come from a trusted source.
https://www.juniper.net/documentation/us/en/software/junos/denial-of-service/topics/topic-map/security-introduction-to-adp.html

This is necessary to ensure that the application firewall can properly identify the application and the correct security policies can be applied before allowing any traffic to pass through.
If the first packet was allowed to pass without first being identified, then the application firewall would not know which security policies to apply - and this could potentially lead to security vulnerabilities or breaches. So it's important that the first packet is held until the application is identified.

User-defined security zones allow users to configure multiple security zones and share them between routing instances. This allows users to easily manage multiple security zones and their associated policies. For example, a user can create a security zone for corporate traffic, a security zone for guest traffic, and a security zone for public traffic, and then configure policies to control the flow of traffic between each of these security zones. Transit traffic can also be managed using user-defined security zones, as the policies applied to these zones will be applied to the transit traffic as well.