The grammar rules for a language determine only the syntax. The semantics are determined by the semantic values associated with various tokens and groupings, and by the actions taken when various groupings are recognized. Multiple Types Specifying several alternative data types. An action is the semantic definition of a grammar rule. Action Types Specifying data types for actions to operate on.
Most actions go at the end of a rule.
This says when, why and how to use the exceptional action in the middle of a rule.