Wikia

Senior Chief Petty Officer is the eighth enlisted rank in the Federation Starfleet. The rank is higher than aChief Petty Officer and lower than a Master Chief Petty Officer. The rank is the second highest that can be attained by a non-commissioned Starfleet officer. Personnel having this rank are referred to as Senior Chief in most circumstances.

Advancement to the rank of Senior Chief Petty Officer is made in the same way as that of Chief Petty Officer. It bears requirements of time served in Starfleet, superior evaluation scores by superior officers and Commanding Officers, and a review of peers. It is the first promotion that is based entirely on proven leadership performance where examination scores do not play a part. A Chief Petty Officer can only advance to the rank of Senior Chief Petty Officer if a board of Master Chiefs approve. This happens once every year.

As with Chief Petty Officers, a Senior Chief Petty Officer takes on more administrative and command duties.