Charles Ellicott Commentary Numbers 34:2-3

"Command the children of Israel, and say unto them, When ye come into the land of Canaan (this is the land that shall fall unto you for an inheritance, even the land of Canaan according to the borders thereof), then your south quarter shall be from the wilderness of Zin along by the side of Edom, and your south border shall be from the end of the Salt Sea eastward;" — Numbers 34:2-3 (ASV)

When you come... — Better, You are entering into the land of Canaan; this is the land which shall fall to you for an inheritance, (even) the land of Canaan, according to its borders. And your south quarter (or, district) shall be from the wilderness of Zin by the side of Edom; and your south border shall be from the extremity of the Salt Sea eastward (or, on the east).

It was important for the Israelites to be taught that, while divinely commissioned to exterminate the idolatrous inhabitants of the land of Canaan, they had no commission to make aggressive wars upon the surrounding nations which were beyond the confines of the land which was allotted to them. The southern boundary which is here described is the same as that of the tribe of Judah, as described in Joshua 15:1-2. The land of the Israelites was to extend towards the south as far as the wilderness of Zin, which was to divide their territory from that of the Edomites.