The meaning is, “and David took all the sheep and oxen which the Amalekites drove” (i. e. had in their possession) “before that acquisition of cattle” (namely, before what they took in their raid to the south), “and they” (the people) “said, This is David’s spoil.” This was his share as captain of the band (compare Judges 8:24-26). All the other plunder of the camp - arms, ornaments, jewels, money, clothes, camels, accoutrements, and so on - was divided among the little army. David’s motive in choosing the sheep and oxen for himself was to make presents to his friends in Judah 1 Samuel 30:26-31.
Related Commentaries of 1 Samuel 30:20
1 Samuel 30:20
20 And David took all the flocks and the herds, which they drave before those other cattle, and said, This is David's spoil.