وَحَرَّمَ الرِّبَا
“And He has prohibited riba.” (Al-Baqarah 2:275, relevant phrase)
The prohibition is plainly stated. Yet identifying the precise legal content of riba and its different forms requires further authoritative explanation. The book uses that second aspect as an illustration of mujmal.
This distinction is essential: the fact of prohibition is not unclear. The problem is deciding which kinds of excess or transactions belong to the technical legal category. Thus a text may be clear as to its core command while needing explanation for detailed application.