As volunteers and auxiliaries, to be ready upon occasion, as the service of God and the king should require. Possibly these or most of them were the strangers which had come out of Israel into the kingdom of Judah in Asa's days, and probably since that in his time.
Related Commentaries of 2 Chronicles 17:16
2 Chronicles 17:16
16 And next him was Amasiah the son of Zichri, who willingly offered himself unto the LORD; and with him two hundred thousand mighty men of valour.