When a security policy is deleted, the active sessions allowed by the policy will be dropped. The default behavior is that all active sessions allowed by the policy will be terminated and the traffic will no longer be forwarded. There is no way to mark the active sessions as a legacy flow or to reevaluate them by the cached rules.
Reference:
According to Juniper Networks Security, Specialist (JNCIS-SEC) Study Guide, when a security policy is deleted, the active sessions allowed by that policy will be dropped. This behavior is the default behavior of the device. There is no way to mark the active sessions as a legacy flow or to re-evaluate them against cached rules. The device will terminate the active sessions and will no longer forward traffic for those sessions.

The fab interface is a fabric link that connects the two nodes in a chassis cluster. A chassis cluster is a high-availability feature that groups two identical SRX Series devices into a cluster that acts as a single device. The fab interface has two functions:
Real-time objects (RTOs) are exchanged on the fab interface to maintain session synchronization: RTOs are data structures that store information about active sessions, such as source and destination IP addresses, ports, protocols, and security policies. RTOs are exchanged between the nodes on the fab interface to ensure that both nodes have the same session information and can take over the traffic in case of a failover.
In an active/active configuration, inter-chassis transit traffic is sent over the fab interface: In an active/active configuration, both nodes in a cluster can process traffic for different redundancy groups (RGs). RGs are collections of interfaces or services that fail over together from one node to another. If traffic needs to transit from one RG to another RG that is active on a different node, it is sent over the fab interface.

Explanation
Juniper Secure Analytics (JSA) is a security information and event management (SIEM) system that consolidates, analyzes, and manages surveillance data from network devices, endpoints, and applications1 JSA uses two features to configure alerts based on certain criteria: building blocks and events2 Building blocks are reusable components that define common characteristics of network activity, such as IP addresses, ports, protocols, usernames, or threat categories. Building blocks can be used to create custom rules, searches, reports, and filters that can trigger alerts when certain conditions are met2 Events are records of network activity that are collected and normalized by JSA. Events can be classified into different categories, such as offenses, flows, logs, or anomalies. Events can also be correlated with other data sources, such as vulnerability scanners, threat intelligence feeds, or asset databases, to provide more context and insight. Events can trigger alerts when they match predefined or custom rules that specify the severity, frequency, or duration of the activity2 References: 1: JSA Series Secure Analytics - Juniper Networks 2: Juniper Secure Analytics Users Guide

Explanation
Juniper ATP Cloud is a cloud-based threat detection service that protects all hosts in your network against evolving security threats. It uses a combination of tools to identify and block malware, such as cache lookup, static analysis, and dynamic analysis12 Cache lookup is the first step in the malware detection process. It checks the file hash against a database of known malicious and benign files. If the file is found in the cache, the analysis is completed and the file is either allowed or blocked based on the cache result12 Static analysis is the second step in the malware detection process. It examines the file attributes, such as file size, file type, file name, and embedded URLs, to determine if the file is suspicious or not. If the file is deemed suspicious, it is sent to the next step for further analysis. If the file is deemed benign, it is allowed to pass through12 Dynamic analysis is the third and final step in the malware detection process. It executes the file in a sandbox environment and observes its behavior, such as network connections, registry changes, file operations, and process injections. If the file exhibits malicious behavior, it is blocked and reported. If the file does not exhibit malicious behavior, it is allowed to pass through12 Therefore, dynamic analysis is not always performed to determine if a file contains malware, as it depends on the results of the cache lookup and static analysis. Similarly, if the cache lookup determines that a file contains malware, static analysis is not performed to verify the results, as the file is already blocked based on the cache lookup12 References:
Juniper Advanced Threat Prevention Cloud (ATP Cloud) Documentation
Juniper Advanced Threat Prevention Cloud | Juniper Networks