In the book’s discussion of binding authority, the qualification «في فروع الدين» directs attention to practical legal branches on which qualified jurists agree.
Distinguish two tasks:
Deriving a ruling: interpreting the Qur’an, Sunnah and legal principles.
Establishing consensus: showing that the relevant jurists of that period actually agreed.
Finding a ruling in a respected manual does not, by itself, prove ijma‘. The ruling may be sound, but claiming unanimous agreement is a further evidentiary claim.