This layer of information is the indication of the formal category of the non-terminal syntactic units or constituents identified by bracketing, such as Noun Phrase, Verb Phrase, Relative Clause. For example, adding labels to the above string yields the labelled analysis of 4:
(4) | [S [NP He NP] [VP walked [PP into [NP the garden NP] PP] VP] S] |
(5) |