What is the meaning of "l'Negdam"?
'Rashi': It is by them, like "v'Elechah l'Negdecha."
Who was in the rest of Bashan?
Radak: "V'Yeser ha'Gil'ad v'Chol ha'Bashan? Nasati la'Chetzi Shevet ha'Menasheh." The minority of it, until Salchah, was of Gad. The Torah said that all of Bashan was to Menasheh, i.e. what was from Og's kingdom - all Chevel Argov, which was called Eretz Refa'im. Even though he was called Melech ha'Bashan, a minority of Bashan was not his, rather, of the Emori, i.e. until Salchah. It says "va'Yeshevu ba'Gil'ad ba'Bashan uvi'Vnoseha" (16) refers to the Bashan until Salchah. Chetzi Menasheh dwelled in the Bashan until Ba'al Chermon (23). In Yehoshua (22:7), it says "vela'Chatzi Shevet ha'Menasheh Nasan Moshe Bashan" - it does not say Kol ha'Bashan 1 !
Malbim: Initially, it was totally of Menasheh. Later, they made a new division, and Gad received part, until Salchah.
There, it teaches that half the Shevet received east of the Yarden, and half received in the west. Earlier (13:30) it says that Menasheh received Kol ha'Bashan! (PF)
