Bracketing is always accompanied by the indication of the syntactic category of the phrasal constituent (e.g. Noun Phrase, Verb Phrase, Relative Clause) enclosed between brackets. Unlabelled bracketing is only adopted in cases of unrecognized syntactic constructions, i.e. when a label cannot be assigned to a constituent; Lancaster/IBM, TOSCA and U.Penn are annotation schemata which resort to this practice.
Adding syntactic labels to the bracketed sentence above yields the following labelled analysis:
[S [NP He NP] [VP talked [PP to [NP the guardian NP] PP] VP] S]
The labelled bracketing representation encodes the same information as a tree structure but presents it linearly.
As to the typology of phrasal constituents, there is a certain amount of variability among different annotation practices. Nonetheless, the EAGLES Syntactic Annotation Group isolated a subset of syntactic categories which are, with qualification, subsumed by most current practices of constituency-based syntactic annotation:
Since dependency-based annotation schemata recognize only word-level categories, they clearly do not include this layer of information.