From Star Trek: Theurgy Wiki
Starfleet Intelligence was a branch of Starfleet responsible for gathering and analyzing information concerning neighboring powers in the galaxy, and for carrying out covert investigations regarding possible threats to Earth and, later, the Federation.
Starfleet Intelligence employed agents for their field operations who were authorized to perform undercover investigations. Starfleet Intelligence agents (or "SI agents") reported to division heads who oversaw their agents' missions. The head of the organization was known as the Chief of Starfleet Intelligence.
One branch of Starfleet Intelligence was the Department of Internal Affairs, which was charged with keeping track of Starfleet operations to ensure that no security leaks occurred.
An Intelligence officer is a title given to an officer aboard a starship or space station who is responsible for intelligence matters. The officer typically keeps in contact with a central organization, such as Starfleet Intelligence, for the latest information.
Aboard Deep Space 9, Lieutenant Commander Worf held the position upon accepting assignment there. Doctor Julian Bashir briefly assumed the post in late-2373, while Worf was serving on the IKS Rotarran. It could only have been Bashir or Jadzia Dax, as only Starfleet officers with a class four security clearance could hold the position. He told Chief Miles O'Brien "All I can do is read these fascinating reports and analyses, and analyses of analyses and then keep it all to myself. Because no one else has a 'need to know'" Source: Memory Alpha