This setting provides several availability options for the Lacros browser.
If the policy is set to user_choice, the user can enable Lacros and make it primary.
If the policy is set to lacros_disallowed, the user cannot use Lacros.
If the policy is set to side_by_side, Lacros is enabled but is not the primary browser.
If the policy is set to lacros_primary, Lacros is enabled and is the primary browser.
If the policy is unset, the default is lacros_disallowed for enterprise-managed users and user_choice for non-managed users.
In the future it will be possible to make Google ChromeOS with lacros_only value.