If they accept your offer of peace and open their gates, all the people there will become forced laborers to serve you.
They are here directed what method to take in dealing with the cities (these only are mentioned, Deu 20:10, but doubtless the armies in the field, and the nations they had occasion to deal with, are likewise intended) upon which they made war.
Commenting on Deuteronomy 20:10-20
And it shall be, if it make thee answer of peace,.... Comply with the terms of peace offered: and open unto thee; the gates of the city and its garrisons, and deliver all into their hands: then it shall be that all the people that is found therein; some having made their escape before the surrender of the city: shall be tributaries unto thee: pay...
And it shall be, if it make thee answer of peace, and open unto thee, then it shall be, [that] all the people [that is] found therein shall be tributaries unto thee, and they shall serve thee. (e) If it accept peace.