The senior staff of a starship or starbase generally consists of the commanding officer and other senior crew members who hold various positions of authority aboard that particular vessel or base.
Those included on the senior staff are at the discretion of the CO and will vary in rank and position, depending on staff available. For example Captain Jean-Luc Picard elected not to have a science officer on his senior staff aboard the USS Enterprise-D, with science-related matters being dealt with by senior operations officer, Lieutenant Commander Data. Similarly, Captain Benjamin Sisko of Deep Space 9 did not include a counselor on his senior staff.
Rank is not always a factor in determining one's inclusion on the senior staff. For example, an ensign serving as senior operations manager may be included on the senior staff, whereas a lieutenant commander serving as a gamma shift watch officer may not. Further a non-commissioned officer of sufficient experience may be a member of the senior staff if the situation dictates.