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. In the Federation Starfleet, 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 starship USS Enterprise-D (and E), with science-related matters being dealt with by senior operations manager, Lieutenant Commander Data. Similarly, Captain Benjamin Sisko of Deep Space 9 did not include a counselor on his senior staff.