In 2357, a group of Rangers were stationed on the USS Icarus for the Arias expedition. Each was scientifically trained and able to aid the ship in its primary mission, the study of massive compact halo objects. However, the Rangers had the additional mission of gathering intelligence on the Cardassians as the Icarus traveled near their space. The identity of the Rangers was unknown to all but themselves and Admiral Owen Paris.
Six months into the mission, Admiral Paris and Ensign Kathryn Janeway were captured by the Cardassians. Within a short time, the group of Rangers rescued Paris and Janeway. Janeway recognized Justin Tighe, a member of her science team, as among her rescuers. (VOY novel: Mosaic)
One of the unpublished material from Last Unicorn Games included a Starfleet Security Handbook which included information on the Starfleet Rangers. According to this source, they recent innovation in Starfleet with an emphasis on planetary survival skills as well as the ability to work alone for extended periods of time. This allowed them to either work individually or in small groups where they could explore, protect and defend the borders of Federation space.