For example, the aircraft pilot could be the junior member of a flight crew such as a USN lieutenant, junior grade/USMC 1st lieutenant or USN lieutenant/USMC captain, and the weapon systems officer could be a senior officer such as a USN lieutenant commander, commander or captain, or a USMC major, lieutenant colonel, or colonel; this would likely make the WSO the mission commander.
This passage describes a situation where the pilot and weapon systems officer are part of a flight crew with different ranks. The senior officer is likely to be the mission commander.