«النفل لغة: الزيادة، وشرعاً: ما هو زيادة على الفرائض والواجبات، ويقال له: التطوع والمندوب أيضاً»
Lexically, nafl means an addition; legally it is an act beyond fard and wajib. The author also calls it tatawwu or mandub.
An “addition” is an act beyond the basic binding duties, not an act lacking religious worth. Voluntary worship and virtuous deeds may be meritorious within their relevant conditions.
What distinguishes this category is that approval does not amount to a binding duty.