Angled brackets refer to a notational convention that used to collapse rules.
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 <X> Z
The difference between the rules (i) and (ii) is the presence or absence of the variable X.