«وَهُوَ الْوُصُولُ إِلَى شَيْخِ شَيْخِهِ كَذَلِكَ»
“Reaching the teacher of the compiler's teacher by another route.”
Muwāfaqah joins the route at the compiler's own teacher. Badal joins it one generation earlier, at that teacher's teacher.
Example: Al-Bukhārī relates through Qutaybah from Mālik. Another independent route reaches the same Mālik through al-Qaʿnabī instead of Qutaybah. The routes meet at Mālik: this is badal.
The location of the meeting in the chain, not simply the existence of an alternative narrator, determines the term.