Default UCM access for Fusion Users

Hello Experts

We have setup a new user and did not assign any roles. While checking in UCM, user can still access documents from few secuirty groups.

trying to understand how did user get UCM security group access without assigning any role.

Does UCM access for certain security group is by default for all Fusion users ?

if so is there a way to restrcit access for default security groups ?