What does the 's' character mean in iPhone 5s and 6s series

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?

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.

It's debatable.

Some say the "s" always stands for "successor". Like how the 6s makes great improvements on the 6, but since it doesn't have a new design, Apple shouldn't really call it the iPhone 7.

However, Phil Schiller, at the iPhone 3GS release event, said specifically that the S in 3GS stood for "speed", based on how much faster it was than the 3G.

Fans also sometimes make up their own meanings for what the s stands for:

iPhone 4s: Siri

iPhone 5s: Security (for TouchID)

However, at this point, Apple's naming scheme is set after almost a decade and they'll just give every other iPhone name an s, so it likely does just mean successor.