מִשָּׁם֮ נָסָעוּ֒ וַֽיַּחֲנ֗וּ מֵעֵ֤בֶר אַרְנוֹן֙ אֲשֶׁ֣ר בַּמִּדְבָּ֔ר הַיֹּצֵ֖א מִגְּב֣וּל הָֽאֱמֹרִ֑י כִּ֤י אַרְנוֹן֙ גְּב֣וּל מוֹאָ֔ב בֵּ֥ין מוֹאָ֖ב וּבֵ֥ין הָאֱמֹרִֽי׃
From thence they journeyed, and pitched on the other side of the Arnon, which is in the wilderness, that cometh out of the border of the Amorites.—For Arnon is the border of Moab, between Moab and the Amorites;
Rashi on Numbers
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.
Ramban on Numbers
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.
Sforno on Numbers
מעבר ארנון...היוצא, in that region where the river Arnon borders the region where the desert starts.