Verse 1 – The most important category of classification here is that there were 12 divisions composed of 24,000 soldiers each. Each of those units was on duty for one month out of the year during times of peace. During times of war, obviously everyone would be mobile.
Verses 5, 6 – Benaiah was the most honored of the thirty (11:22), and he was in charge of the Philistine mercenaries (18:17).
Verse 17 – One person was in charge of the tribe of Levi at large, but the descendants of Aaron-the priests-were represented separately by Zadok.
Verse 24 – The book of the annals of King David was a historical document kept in the royal archives with other officials records. It no longer exists. See 1 Kings 14:19.
Verses 33, 34 – When Absalom rebelled against David, Ahithophel betrayed David and joined the rebellion. Hushai pretended loyalty to Absalom, and his advice caused Absalom’s downfall. Ahithophel ended his life with suicide (2 Samuel 17:23).