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.
Rank is not always a factor in determining one's inclusion on the senior staff. In another Starfleet example, an ensign serving as senior operations manager may be included on the senior staff, whereas a lieutenant commander serving as a duty 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. In extreme situations, even a non-member of Starfleet may hold a senior staff position in a Federation crew. Seven of Nine was included in Captain Kathryn Janeway's senior staff aboard USS Voyager as a specialist in astrophysics, and Captain Sisko had Bajoran Militia personnel Kira Nerys and Odo in his senior staff. This practice dated back to the Earth Starfleet of the 22nd century when Subcommander T'Pol was Captain Jonathan Archer's executive officer aboard Enterprise NX-01. (TOS, TNG, DS9, VOY, ENT)