In Xcode/Xamarin Studio it's possible to define a deployment target. But that defines a lower limit for the destination OS the app is deployed on.
There is a way to define an upper limit for that target?
For example, I'd like to deploy an app only on devices that have iOS 6.x installed and not for iOS y.x (where y>6).
There is some way in Xamarin Studio or Xcode to define that constraint?
For xcode tha answer is definitely no. Apple encourages the use and fast migration to the latest version of the OS, so you are able to set only an inferior limit. Even if you could make an app that is iOS 6 or below only, they probably wouldn't approve it in the review process.
There are two primary settings used for your targeting your builds:
Base SDK & Deployment Target.
The Base SDK = What are the latest features I want available in this app? So if you want iOS 6 features, just use iOS SDK 6 as base SDK but do not prevent the app from installing on devices running iOS > 6.
The Deployment Target = What is the earliest OS I want to be able to run this app?