法華義疏 8.165

8.165

互顯其文也。又上根唯一人，故但告身子；中根數多，故通告大眾。又欲開身子化物之路所以別告，欲廣發時眾求佛之心故通告也。

It is to mutually illuminate the text. Furthermore, the upper root was only one person, hence he addressed only Śāriputra; the middle roots are numerous, hence he addressed the great assembly generally. Furthermore, wishing to open the path of Śāriputra's transforming living beings, he addressed him individually; wishing to broadly arouse the mind of seeking Buddhahood in the assembly of the time, he addressed them generally.