John Gill Commentary Numbers 31:27

"and divide the prey into two parts: between the men skilled in war, that went out to battle, and all the congregation." — Numbers 31:27 (ASV)

And divide the prey into two parts

Into two equal parts, both the number of the people and of the cattle, when taken:

between them that took the war upon them, and went out to battle ;

the soldiers; and by this it appears that they went voluntarily and cheerfully:

and between all the congregation ;

so that the part of them that went to battle, and of those that stayed in the camp, were alike, and which in process of time became a statute and ordinance, (1 Samuel 30:24 1 Samuel 30:25) (Psalms 68:12) .