Why do i need to set platform in CocoaPods?

I just started working with CocoaPods recently(should have done this a long time ago), and im wondering what should the value of platform be. For instance right now i have platform :ios, 8.0(there is tick marks around the 8.0 but i dont know how to escape them in the editor) which tells me that the minimum version of the code using these dependencies is iOS 8.0. Few questions.

1. Is my interpertation right about platform? If not what does platform really do?
2. Does this mean it will only install on version 8.0? If so how to i make it say version 8.0 and all higher versions?
3. So later on when ios version 9.0 comes out say certain dependencies don't work with ios 9.0. Do I need place platform ios: 9.0 underneath the list of dependencies for 8.0 and add another list of dependencies for 9.0 to work with?

This is enough questions for now. Thanks for your help.

Example of number 3

platform :ios, '8.0'target 'my target' do//list of podsendplatform: ios, `9.0`target `my target` do//list of podsend