The "prey" refers to the captives and live-stock: the "spoil" to the ornaments and other effects.

And they took all the spoil,.... In their cities and castles; and all the prey, both of men and of beasts; all sorts of cattle, oxen, sheep, and asses.

All this booty (shalal, booty in goods), and all the prey in man and beast (malkoach), was brought by the conquerors to Moses and Eleazar and the congregation, into the camp in the steppes of Moab. In Numbers 31:12, שׁבי applies to the women and children who were taken prisoners, מלקוח to the cattle taken as booty, and שׁלל to the rest of the prey.

And they took all the spoil, and all the prey, both of men and of beasts.

31:7-12 The Israelites slew the Kings of Midian. They slew Balaam. God's overruling providence brought him thither, and their just vengeance found him. Had he himself rightly believed what he had said of the happy state of Israel, he would not have thus herded with the enemies of Israel. The Midianites' wicked wiles were Balaam's projects: it was just that he should perish with them, Ho 4:5. They took the women and children captives. They burnt their cities and castles, and returned to the camp.