Labeled bracketing

Labeled bracketing is a way of representing the structure of an expression by writing square brackets ('[' and ']') to the left and right hand side of its component parts, i.e. words or constituents. The brackets carry subscripts, so-called labels, which state the category of the unit in question.

Example
the structure of the sentence the boy may eat apples is written as in (i):

(i) [IP[NP[Det the][N boy]][I'[I may][VP[V eat][NP[N apples]]]]]

The bracket notation is equivalent to the representation by means of a tree structure. The labeled brackets correspond to the nodes in the tree. Thus, a labeled bracketing like (ii) is equivalent to the tree in (iii).

(ii)   [NP [Det the] [N boy]] (iii)	       NP                /  \ Det  N              /      \ the	   boy

Link
Utrecht Lexicon of Linguistics