The T-600 Terminator model that Kyle Reese (Michael Biehn) briefly mentions in the first Terminator movie is retroactively set up in Terminator Salvation. Although Sarah Connor (Linda Hamilton) and her son John Connor (Edward Furlong) avert Judgment Day in 1995, Skynet still rises in 2004 and causes the whole world to turn into a nuclear wasteland, making way for the events of Salvation's dystopian 2018. History repeats itself in the Terminator franchise, always in a different way, and every memory of the apocalyptic future that Kyle Reese brings with him to 1984 inevitably happens in the Salvation timeline.
Before an adult John Connor (Christian Bale) sends Reese back in time and continues the famous time loop, he leads an attack on Skynet and its army of assorted robots. From hydro-bots to Moto-Terminators and even 60-foot-tall "Harvesters", Terminator Salvation builds upon the original film's vision of the future and takes advantage of its setting to reveal the true scope of what Skynet has achieved with its technology. Given that the iconic Arnold Schwarzenegger T-800 Terminator is still a prototype at the time, the humanoid robot that terrorizes the world is the T-600, a bulkier but less versatile killing machine.
As soon as Kyle Reese saves Sarah from the T-800's first murder attempt in James Cameron's original Terminator, he breaks down the robot to her, saying "The 600 series had rubber skin. We spotted them easy, but these are new. They look human — sweat, bad breath, everything." In Salvation, the cyborg Marcus Wright (Sam Worthington) arrives in a dilapidated Los Angeles and sees a distant human figure walking through a street. He yells at it and sees the figure turn around with a Gatling gun before a young Kyle Reese (Anton Yelchin) saves him from its storm of bullets. They escape the T-600 and ultimately crush it.
This scene calls back to the original mention of the robot in a simple, yet dramatic way. It shows how someone who isn't familiarized with the T-600 could confuse the machine with a human being because of its humanoid shape, even though this particular exemplar lacked most of its rubber skin. It also proves how Reese did in fact adapt to Skynet's constantly-evolving inventions as a talented warrior - one that Skynet would have trouble killing (give or take Salvation's plot hole to avoid John Connor and Kyle Reese's death). But mainly, the T-600 served to underline the crucial difference between the T-800 and every other Terminator model: its organic exterior that lets it infiltrate the Resistance and execute their targets from within. That improvement is the stepping stone that lets Skynet send the first T-800 back to 1984 and set Judgment Day in motion.
Terminator Salvation is the only film in the franchise to exhibit the fabled AI-controlled future to its full extent. Similarly to how Kyle Reese became the beacon of hope for John Connor and the Resistance, the T-600 marked the final stride before Skynet achieved its most successful creation and, compared to future models like Dark Fate's deadly Rev-9, it appears to be more effective in leveraging the fight toward Skynet's side.