There are three basic types of Ranks in STO that need to be understood and disambiguated:

Game Rank.

Fleet Rank.

RolePlay/In Character Rank.

Each serves a particular purpose and function within the game and Roleplaying.

A 'Game Rank' (also called Player Rank ) is the title rank bestowed upon a character for having achieved a specific experience level within the game. It is directly tied into game play and reflects the efforts of a player to level up their character. Progressing through the Game Ranks unlocks access to increasingly more effective and powerful ships, weapons, and equipment as well as unlocking further game play content. 

A 'Fleet Rank' is the rank assigned to your character upon joining a fleet in the game. OOCly Fleet Ranks serve in an administrative role to assign varying degrees of access and permissions to fleet assets, such as Starbase Stores and the Fleet Bank. ICly Fleet Ranks serve an important organizational function for seniority and command responsibilities within the Fleet. ( Fleet Ranks in the 8th ETFFleet Ranks in The Swordfighter Squadron)

A “RolePlay/In Character Rank” is the rank you actually RP your character as being. This rank represents the rank you display in titles and with rank insignia on your uniform. ICly this is the most important rank to understand as it represents the rank you will be interacting with other members of the fleet as. (IC Ranks of the 8th ETF , IC Ranks of The Swordfighter Squadron )