It appears that what is meant by "al-Nawadir" are uncommon reports that do not fall under a specific chapter and are dispersed. The term may also refer to rare or unusual narrations.
الظاهر أن المراد بالنوادر التي لا يجمعها باب وتكون متفرقة، وقد يطلق على الاخبار الشاذة.