Angled brackets

Angled brackets refer to a notational convention that used to collapse rules.

Example
The two rules (i) and (ii) can be collapsed as (iii).

(i)	A -> B /_____Y X  Z

(ii)	A -> B /_____Y Z

(iii)	A -> B /_____Y  Z

The difference between the rules (i) and (ii) is the presence or absence of the variable X.

Link
Utrecht Lexicon of Linguistics