From Destinypedia, the Destiny wiki
Frames are multi-purpose non-sentient robots that serve the City and Reef. They are mainly used in civil or defensive roles by both powers. In Destiny 2 some Frames were saved during the invasion of the Tower. These Frames are defending The Farm in the European Dead Zone. During the Battle of the Twilight Gap, Shaxx's Redjack Frames had suffered major casualties. During the Quest to create Devil's Ruin, The Guardian collected the Frames to find the correct pieces.
Developed during the Golden Age, but not nearly as advanced as the fully sentient Exo, frames are useful for both civil and military applications. Some frames are found in the Tower as vendors; other frames are used as defense robots. Lord Shaxx uses platoons of frames called Redjacks to secure potential Crucible sites. Frames are capable of learning from experience, developing unique personalities over time.
The Reef also uses frames as auxiliary guards, as they can be found wandering the Vestian Outpost and serving similar posts as their Tower counterparts. However, these frames are more primitive, as they lack learning capabilities or personalities, and are wiped from time to time.
There appear to be two model series of frames: the 55-30 series and the 99-40 series. The former is distinguished by their flat-topped heads and small, circular eye, while the the latter feature a more typical head shape with a larger, square eye, as well as two secondary eyes mounted externally to the sides of the head on models produced circa the Red War. It is unknown what these differences signify, however 55-30 frames are most notably functioning as vendors whereas 99-40 frames are commonly seen or implied to serve in maintenance and combat roles.
Forces of the Vanguard's Frames formed the bulk of the Last City's forces during the conflict with the resurgent Hidden Swarm on Luna. The battle near the Hellmouth reached a stalemate until the Guardian's arrival. Upon driving the Hive back, the Vanguard managed to form a base of operations outside Sorrow's Harbor. The Frames remain guarding the newly established outpost, Sanctuary, and its overseer, Eris Morn.
Season of the Worthy's Seraph Tower public event introduced a new variant of Frame built by Rasputin called Heavy Frames, which can be spawned in during the activity to aid Guardian forces by players with certain Bunker upgrades unlocked. In addition regular Rasputin frames also appear in bunkers and in the mission "Into the Mindlab".
In the Bunker E15 lost sector on Europa There are several frames that are trapped in some sort of Vex cage. Releasing them results in them assisting you in completing the lost sector.
Tower Frame Quotes