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
1 Chronicles 9:1
ch1 9:1ORIGINAL REGISTERS OF ISRAEL AND JUDAH'S GENEALOGIES. (1Ch. 9:1-26)
all Israel were reckoned by genealogies--From the beginning of the Hebrew nation, public records were kept, containing a registration of the name of every individual, as well as the tribe and family to which he belonged. "The book of the kings of Israel and Judah" does not refer to the two canonical books that are known in Scripture by that name, but to authenticated copies of those registers, placed under the official care of the sovereigns; and as a great number of the Israelites (Ch1 9:3) took refuge in Judah during the invasion of Shalmaneser, they carried the public records along with them. The genealogies given in the preceding chapters were drawn from the public records in the archives both of Israel and Judah; and those given in this chapter relate to the period subsequent to the restoration; whence it appears (compare Ch1 3:17-24) that the genealogical registers were kept during the captivity in Babylon. These genealogical tables, then, are of the highest authority for truth and correctness, the earlier portion being extracted from the authenticated records of the nation; and as to those which belong to the time of the captivity, they were drawn up by a contemporary writer, who, besides enjoying the best sources of information, and being of the strictest integrity, was guided and preserved from all error by divine inspiration.
1 Chronicles 9:2
ch1 9:2the first inhabitants that dwelt in their possessions--This chapter relates wholly to the first returned exiles. Almost all the names recur in Nehemiah (Neh. 11:1-36), although there are differences which will be explained there. The same division of the people into four classes was continued after, as before the captivity; namely, the priests, Levites, natives, who now were called by the common name of Israelites, and the Nethinims (Jos 9:27; Ezr 2:43; Ezr 8:20). When the historian speaks of "the first inhabitants that dwelt in their possessions," he implies that there were others who afterwards returned and settled in possessions not occupied by the first. Accordingly, we read of a great number returning successively under Ezra, Nehemiah, and at a later period. And some of those who returned to the ancient inheritance of their fathers, had lived before the time of the captivity (Ezr 3:12; Hag 2:4, Hag 2:10).
1 Chronicles 9:18
ch1 9:18the king's gate--The king had a gate from his palace into the temple (Kg2 16:18), which doubtless was kept constantly closed except for the monarch's use; and although there was no king in Israel on the return from the captivity, yet the old ceremonial was kept up, probably in the hope that the scepter would, ere long, be restored to the house of David. It is an honor by which Eastern kings are distinguished, to have a gate exclusively devoted to their own special use, and which is kept constantly closed, except when he goes out or returns (Eze 44:2). There being no king then in Israel, this gate would be always shut.