במדבר כא:יג פירוש: רש״י, רמב״ן, ספורנו וSiftei Chakhamim

מגבל האמרי The word גבול denotes boundary-line of (marking) the end of their frontier. Similar is the term the גבול of Moab (in this verse): it is an expression for extremity and end.

FOR ARNON IS THE BORDER OF MOAB, BETWEEN MOAB AND THE AMORITES. Arnon was a city which belonged to Moab, [located] near the end of the Amorite border and the beginning of the border of Moab, and it was situated on brooks that flowed through it.²⁵⁸Verse 14: and Arnon among the brooks.

מעבר ארנון...היוצא, in that region where the river Arnon borders the region where the desert starts.