仁王經疏 3.13

3.13

問：四眾等已攝比丘，何須別說百比丘耶？為顯一一會中定須一人而為上座，故言百也。

Question: Since the four assemblies and so on already include the monks, why is it necessary to speak separately of the one hundred monks? [Answer:] In order to show that in each assembly there must definitely be one person acting as the senior elder (sthavira); therefore it says 'one hundred.'