What does «فَفِيهَا لُغَاتٌ، أَفْصَحُهَا ...» establish?

Al-Murakkabāt — Compound Numerals, the Twelve Exception and Declining Baalbek

• Only one usage is possible
• Twelve never declines
• There are variants, and the author selected the most eloquent
• Baalbek always has tanwīn