Most textbooks state that subject relative pronouns cannot be omitted, e.g.
A: Alan threatened Brian.
B: Alan was a gangster.
A+B = Alan, who was a gangster, threatened Brian.
However, sometimes I will see the following:
A+B = Alan, a gangster, threatened Brian.
Is this standard usage? If so, is it still a relative clause or something else entirely?
Many thanks in advance.