A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
num 36:1THE INCONVENIENCE OF THE INHERITANCE. (Num 36:1-13)
the chief fathers of the families of the children of Gilead--Being the tribal governors in Manasseh, they consulted Moses on a case that affected the public honor and interests of their tribe. It related once more to the daughters of Zelophehad. Formerly they had applied, at their own instance, to be recognized, for want of male heirs in their family, as entitled to inherit their father's property [Num 27:1-11]; now the application was made on behalf of the tribe to which they belonged--that steps might be taken to prevent the alienation of their patrimony by their alliance with husbands of another tribe. The unrestricted marriages of daughters in such circumstances threatened seriously to affect the tenure of land in Israel, as their inheritance would go to their children, who, by the father's side, would belong to another tribe, and thus lead, through a complication of interests and the confusion of families, to an evil for which even the Jubilee could not afford a remedy. [See on Lev 25:13].
num 36:5Moses commanded the children of Israel according to the word of the Lord--The plea appeared just and reasonable; and, accordingly an enactment was made by which the daughters of Zelophehad, while left to the free choice of their husbands, were restricted to marry not only within their own tribe, but within the family of their father's tribe--that is, one of their cousins. This restriction, however, was imposed only on those who were heiresses. The law was not applicable to daughters in different circumstances (Ch1 23:22) --for they might marry into another tribe; but if they did so, they were liable to forfeit their patrimonial inheritance, which, on the death of their father or brothers, went to the nearest of the family kinsmen. Here was an instance of progressive legislation (see also Exo 18:27) in Israel, the enactments made being suggested by circumstances. But it is deserving of special notice that those additions to, or modifications of, the law were confined to civil affairs; while the slightest change was inadmissible in the laws relating to worship or the maintenance of religion.
num 36:13These are the commandments and the judgments, which the Lord commanded by the hand of Moses unto the children of Israel in the plains of Moab--The Israelitish encampment was on an extensive plateau north of the Arnon, which, though wrested from the Moabites by Sihon and Og, still retained the name of its original possessors. The particular site, as indicated by the words "Jordan near Jericho," is now called El-Koura--a large plain lying not far from Nebo, between the Arnon and a small tributary stream, the Wael [BURCKHARDT]. It was a desert plain on the eastern bank, and marked only by groves of the wild, thorny acacia tree.