The Tal Diann is Military Intelligence, as opposed to the Tal Shiar, which is civilian. The Tal Diann maintains and monitors relations between the Star Command and the various fleets: Command Fleets, Senatorial Fleets, Coalition Fleets, Praetorian Fleets. It officers, Legates, maintain a smooth operational atmosphere throughout the Navy, no easy task considering the disparate nature of the various fleet structures. Each fleet (and wing inside each fleet) has a legate permanently attached to it as an observer.

Legates receive their power directly from Star Command and the Praetor, and as such posses nominal jurisdiction over ALL the orders and fleet movements. In reality, political considerations often take center stage, and the Tal Diann is considered one of the most "comfortable" postings in the Star Navy. Many officers with future political aspirations choose this division for their careers.

The Tal Diann plays a critical role during times of crisis or war. In threat situations or protracted military campaigns, Legates oversee operational and power balances between fleets not accustomed to working together. In such cases, the Star Command establishes temporary "field hierarchies" and non-standard command chains; while commanders in the field may be loath to honor such field alterations and brevet promotions, the Legates attached to their vessels or units enforce them. Officers from the Tal Diann monitor all activities within these temporary coalition and command fleets, reporting any unusual or suspicious events or orders to the Star Command.

In addition to policing the ranks of the military, the Tal Diann serves as an intelligence branch attached to the Romulan military. While the Tal Diann does not wield the same level of power as the Tal Shiar, it commands formidable respect within the military. On a more global level, the Tal Diann and the Military Affairs branch of the Tal Shiar often find themselves at odds with each other. Neither organization would mourn the downfall or the political destruction of the other.