Commentary for Numbers 2:5

והחונים עליו, “and those encamped next to him” (Yehudah). Five of Leah’s sons were among the twelve tribes that were each distinguished by flags. Two of Leah’s flags tribes shared the camp of Gad, who himself was the firstborn of Zilpah, Leah’s handmaid who cooperated closely with the camp of Reuven who was the firstborn of all the tribes. One flag was for Yehudah and the two tribes that were from the sons of the handmaids. Two of the handmaids’ sons who were not firstborns, camped alongside him.