Komentarz do Diwrej ha-jamim I 18:15

Now Joab, etc. was over the host Since he retired from going with the army, he placed Joab over the host to wage all his battles. All this refers back to “... and he administered justice and charity.” The interruption with “Now Joab, etc. was over the host,” was necessary for the following reason, lest you say that since “... and David was administering justice,” [the result was that] the Israelites were no longer waging war, therefore, it says “Now Joab was over the host.”

recorder - Heb. הַמַּזְכִּיר, lit., the reminder. This refers back to the beginning (verse 14), that he would remind [him of] which case came before the king first, and a similar instance is (Esther 6:1): “... to bring... the records (הַזִּכְרוֹנוֹת), the chronicles.”