What we have to consider if designing an iPhone app supporting iOS version 9? [on hold]

Requirement: Design an iPhone to-do (daily task manager) app supporting iOS version 9.

Could you please explain here in more details what I have to consider if designing an App for iOS version 9? I found this article as well:https://designcode.io/iosdesign-guidelines

Does it mean that design just has to reflect/follow iOS 9 Design Guidelines for iPhone/iPad and has to be compatible? Am I right?

I would grateful for detailed explanation. Thanks.