I found a similar question for iPhone 3Gs and iPhone 4s here. So what about the 's' character meaning in 5s and 6s series of iPhone?
IPhone – What does the ‘s’ character mean in iPhone 5s and 6s series
iphone
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Best Answer
The iPhone 5s "s" apparently stands for security (they added touch ID), but the "s" in iPhone 6s is still largely debated, though some have speculated that it might be for sensitivity (as per the new 3D Touch features)
However, apple has never really released any official information regarding what the s stands for in any models other than the 3GS and 4s.
But really, I don't think apple plans what the s stands for, it is just a symbol to show that this model has improvements upon its predecessor, but is not altogether different.