Computed columns and function-based indexes differ in some respects.
A computed column provides both shorthand for an expression and indexability, while a function-based index provides no shorthand; it allows you to index the expression directly.
A computed column can be either deterministic or nondeterministic, but a function-based index must be deterministic. “Deterministic” means that if the input values in an expression are the same, the return values must also be the same.