(I don't know the grammar for rule attributes in CAST.) In HAC-PRS, production rules may be tagged with an arbitrary number of attributes.
The most common example is the after delay attribute, which was written in CAST:
after 100 p -> q+
which would now be written in HAC:
[after=100] p -> q+
Attributes appear as a semicolon-delimited, square-bracket-enclosed list of key-value pairs prefixing the rule. An example of multiple attributes:
[after=100;weak=1] p -> q+
Attributes that appear before a loop-enclosed rule will apply to all iterations of the looped rule. The expressions in attributes may be parameter-dependent or induction-variable dependent, resulting in different values per iteration.