«الفرق: هو بيان الفرق بين الأمرين»
Farq identifies a material difference between the two cases being compared.
The textbook uses a dispute over zakat on wealth belonging to a minor as compared with an adult. The reply points to a difference relevant to the asserted legal purpose or conditions.
Not every observable difference invalidates qiyas. The distinction must matter to the legal cause or to the ruling that is being transferred.