وَمِنْهَا: «حَيْثُ» بُنِيَتْ تَشْبِيهًا لَهَا بِالْغَايَاتِ؛ لِمُلَازَمَتِهَا الْإِضَافَةَ إِلَى الْجُمْلَةِ فِي الْأَكْثَرِ.
Translation: Another is ḥaythu, treated as indeclinable by analogy to ghāyāt because it is usually linked to a whole clause.
Ḥaythu introduces a location, normally followed by a clause. The source explains its indeclinability by analogy with ghāyāt and its frequent attachment to clauses.