Halliday & Matthiessen (2014: 470n):
As with other cases of hypotactically dependent clauses that are non-finite and lack an explicit conjunctive marker, the logico-semantic relationship may be somewhat indeterminate; in the case of the intensive relational clause consisting of Attribute only, without an explicit Process, there may be a causal feature of enhancement, e.g. A Shi’a Muslim, Mr Sahhaf is an outsider in the Sunni-dominated government that has been in power since 1968 (‘since he is a Shi’a Muslim, Mr Sahhaf is an outsider ...’). Compare thematic circumstances of Role with a temporal connotation: As a child she lived at Herne Bay (‘when she was a child, she lived ...’).