How work subtitle options "Enable burned-in subtitle"?

Hi, I want to know how exactly work this setting? What is the difference between Always and only if required? What is the condition for "only when required"?
Because when I specified fre as prefered sub language and I select "only when required" the sub are not show. But If I choose Always, that's working...
Thanks you.
Because when I specified fre as prefered sub language and I select "only when required" the sub are not show. But If I choose Always, that's working...
Thanks you.