כָּל־אֵ֙לֶּה֙ חָֽבְר֔וּ אֶל־עֵ֖מֶק הַשִּׂדִּ֑ים ה֖וּא יָ֥ם הַמֶּֽלַח׃
All these came as allies unto the vale of Siddim—the same is the Salt Sea.
Rashi on Genesis
עמק השדים VALE OF SIDDIM — So it was named because it contained many fields (שדות)
Sforno on Genesis
כל אלה, all the ones just named, Amrafel and the two groups that conducted the war.