the code talks about concubines and wives alike. how were they different? what rights did each have

Check out the excerpt below:

Concubinage was then and theretofore, as now, also an institution in China and is recognized by Confucius and rules laid down also for its regulation. The relationship was treated as not less regular than that of marriage but it involved lower standing for the concubine and her offspring; notwithstanding which frequently the wife's younger sister became the concubine, not without the active connivance of the wife, lonely amid unfamiliar surroundings and longing for the companionship of her own kin. The wife had dominion in the home over concubines and their children.

The double standard was therefore known and its consequences openly accepted, though in the majority of homes one wife reigned supreme and, as has been seen, it was such a home the felicity of which Confucius portrayed in his tribute to the marriage relation, quoted at the close of the next preceding subdivision.

Concubinage was deemed not merely permissible but commendable when the wife remained barren or even when there were daughters but no son to perpetuate the name of the husband and maintain the altars of devotion of his ancestors. Had it been otherwise, undoubtedly divorces, with their hardships, would have been more common.