What command prevents designated units from starting, including during system boot?

The command that prevents designated units from starting, including during system boot, is indeed the one that masks units. When you use the command systemctl mask, it creates a symbolic link from the unit file to /dev/null. This effectively disables the service completely by making it so that it cannot be started manually or automatically by the system, regardless of the desired runlevel or target. This is particularly useful for services that you want to ensure are never running, as it guards against any attempts to start them, either via other services' dependencies or during boot.

In contrast, other systemctl commands have different functions. For example, systemctl stop only stops a currently running service but does not prevent it from starting again on the next boot. systemctl disable prevents a service from starting at boot by removing its symlink in the system's default target, but the service can still be started manually. Finally, systemctl unmask is used to reverse the masking, allowing the service to start again. Thus, masking is the most definitive way to ensure a unit does not start at all.