* If you want to only support iOS (any version): `s.platform = :ios`
* If you want to only support iOS from 6.1 and up: `s.platform = :ios, '6.1'`. Or when not using the deployment target shortcut that the `platform` attribute offers: `s.platform = :ios; s.deployment_target = '6.1'`.
* If you want to support both iOS and OS X (any version) then you don’t need to set a `platform` value at all (i.e. the attribute limits).
* If you want to support both iOS and OS X from specific versions up, then don’t set a `platform` value, but do set the `deployment_target` for both platforms.

http://guides.cocoapods.org/syntax/podspec.html#deployment_target