Explanation
The correct upgrade method for upgrading from R80.40 to R81.20 without any downtime is the Multi-Version Cluster Upgrade (MVC). MVC is a new feature in R80.40 that replaces the deprecated Connectivity Upgrade (CU). MVC allows you to upgrade cluster members to a newer version without losing connectivity and test the new version on some of the cluster members before you decide to upgrade the rest of the cluster members.
MVC synchronizes connections between cluster members that run different versions and ensures that the cluster remains operational during the upgrade process. MVC is intended only to test the current configuration in the newer version and not to change the security policy and install it on cluster members with different software versions. MVC is disabled by default and can be enabled on each cluster member individually. MVC has some limitations, such as not supporting VSX clusters, IPS blade, or SecureXL acceleration.
References:
Multi-Version Cluster (MVC) replaces Connectivity Upgrade (CU) in R80.40 Multi-Version Cluster (MVC) Upgrade Configuring the Multi-Version Cluster Mechanism

The default shell of Gaia CLI is clish. Clish stands for Command Line Interface Shell and it is a restrictive shell that controls the number of commands available in the CLI. Clish provides a user-friendly interface that supports command completion, history, and help functions. Clish also supports role-based administration, which means that different users can have different levels of access to Gaia features and commands based on their roles.

Explanation
UserCheck is a communication tool used to inform a user about a website or application they are trying to access. UserCheck allows administrators to interact with users in real time, informing them of the security policy and the actions they need to take. UserCheck can also enable users to self-remediate incidents or request exceptions from the administrator. References: Training & Certification | Check Point Software, Check Point Resource Library

Explanation
For Stateful Mode configuration, chain modules marked with 2 will not apply. Stateful Mode configuration is a feature that allows administrators to define how packets are processed by different firewall kernel modules in inbound and outbound directions. Chain modules are firewall kernel modules that perform various security functions, such as VPN, IPS, QoS, etc. Each chain module is associated with a key, which specifies the type of traffic applicable to the chain module. The key can be one of the following values: 0 for all packets, 1 for stateful packets, 2 for stateless packets, and 3 for no match packets. For Stateful Mode configuration, only chain modules with key 0 or 1 will apply, as they handle all packets or stateful packets. Chain modules with key 2 will not apply, as they handle stateless packets, which are not relevant for Stateful Mode configuration.