What is the difference between S-attributed and L-attributed definitions ?

Aspect	S-attributed Definitions	L-attributed Definitions
Order of attribute evaluation	Attributes are calculated as we traverse the parse tree from leaves to root, allowing attributes to depend on children and siblings.	Attributes are evaluated in a single pass from root to leaves, with restrictions on which attributes can depend on each other.
Dependency restrictions	No strict rules on attribute dependencies.	Some rules limit where attributes can get their values from, especially for inherited attributes.
Evaluation strategy	Typically uses bottom-up evaluation, computing synthesized attributes as we move up the tree.	Typically uses top-down evaluation, passing inherited attributes down the tree.
Example	In arithmetic expressions, the value of an expression depends on the values of its parts, so it’s S-attributed.	In type checking, the type of an expression might depend on its context, so it’s often L-attributed.

Compiler Design