What is "Al ha'Mas"?
Radak: It is to levy a tax on Yisrael when David needs to pay his soldiers or do his will. It is one of the laws of kingship. Chazal (cnh 20b) learn "v'Atem Tihyu Lo la'Avadim" (Shmuel I, 8:17) from "v'Hayu Lecha la'Mas va'Avaducha" (Devarim 20:11), that he can impose taxes on you.
David's appointees were listed above (8:16-18), and it did not list an officer over taxes!
Radak: That was the beginning of his kingship. He did not yet put a tax on Yisrael.
Malbim citing Mahari: Now he imposed taxes on Yisrael because they rebelled against him.
What is "Mazkir"?
Targum Yonasan: Memuna Al Duchrana. Radak - i.e. he is appointed over the chronicles.
Malbim: He mentions all who come to the king for Mishpat (i.e. so they will be judged in the order in which they came).
