What is the severity of someone's behavior if they are on Section or Level !

Section 2 refers to Section 2 of the Mental Health Act, which is legislation regarding the detention and treatment of people with Mental health Issues in England and Wales.
A Section 2 allows for the compulsory admission of someone into Hospital for observation and treatment for a period of up to 28 days.
So, to answer your question, a person needs to be quite unwell / a big enough cause for concern for a Section 2 to apply to them. So, yeah, it's pretty severe.
In answer to the question you haven't asked, Hospital's probably the best place for them, for now. It means they can start to receive treatment and support.
I hope this helps.

I've worked with people with mental health problems and have some knowledge of relevant legislation and methods of treatment.