Saw this comment from a fan under the Genisys trailer on youtube. Kind of a long explanation but definitely appreciated.
It's not a Reboot, its an Alt Universe/Multiverse, story, if you remember when Kyle Reese (Micheal Beihn) was sent back into the past in Terminator (1984), the Resistance sent him back at the same time Skynet sent back the T800, and the Time Machine malfunctioned! we never saw in that film what the malfunction did, now we do, According to the Synopsis posted by the screenwriter on Denofgeek, the malfunctions created TWO Kyles, as well as TWO "T-800" models, creating a Universe C to go along with the A and B universes created in T1.
In universe A we had Sarah, raised by her parents, and a bit of a wet blanket. (T1), that Sarah, in universe A (T2 Directors cut) became a total badass after Kyles Death, and Stopped Judgement Day in 1997 with the Aid of a T-820.
In universe B, Sarah became the badass as in Universe A, but only Delayed Judgement Day, as Skynet was still created, albeit later, hence the Events of T3 and Sarah Conner Chronicles.
In Universe C, the Universe of Genisys, appropriately set in 1984 , the Skynet of Universe A, as the timeline changed to one where Skynet was never built (it took some time for the timeline change to filter into the future through the time portal), sent back a T-X to Kill Sarah as a child in the 1960s, but the resistance managed to Send back a T850 that saved her, as a result, we see a Sarah Raised by a T-850 meaning she came to her T2 "badass" stage a lot faster, by the events of T1, rather than her only becoming one by the events of T2, it was also he that taught her how to "stop" a T1000. .
The Old Universes (t1-t2 (A)-t3(b)-Chronicles) are still canon, but this shows what happened in the Universe C that was created when the time machine malfunctioned in T1 when Skynet sent the T-800 back at the same time the resistance sent Kyle Reese back.
The whole idea is the resistance (Sarah, Kyle, T850) ensuring Universe C follows the events of Universe A by making sure Skynet is NEVER built, but the Skynet of Universe C is determined to ensure it follows universe B by sending back a T-1000, to create a "double team" with the "clone" T800 created by the time machine malfunction.
A bit complicated. but a cool way to avoid a Reboot, yet still keeping the events of T2 Directors cut, AND t3 and Chronicles canon.
For Aid on Terminator Codes.
T1 (Evil Arnold) is the Original model T-800 Chassis with the Model 101 "Skin Job".
T2 is a T820 Chassis (Faster learning module, dual processors for higher Intelligence and faster target acquisition eyes with improved Chassis motor response speed to match) with Model 101 Skin Job.
T3 is a T850 Chassies (same Improvements as T-820 series, but with Improved Atomic Cell Fuelling allowing for greater overclock on the Chassis motors, allowing for even higher levels of super-strength, as well as a much tougher (and heavier) alloy to be used in the Chassis Skeleton for even higher durability compared to T800 and T820 Codes, closer to a T-400 "Tank" model.), again with Model 101 skin job
T-G is also a T850 Chassies, but with the famous Model 101 Skin job has decayed due to the 20 years the unit has been functioning, resulting in its...Aged..appearance. Nice way to feature the fact that Arnold is now somewhat North of 60!
The fact the T850 is far more advanced than the Evil Arnold T800 model is how it manages to obliterate the cruder, "evil" model with such ease, despite the T800s "younger" appearance, Fresher Skin Job, but Inferior Chassis.