what is plus sign in purchase box in app store
What does the +(plus) sign designate in the upper left corner of the purchase button in the App Store?
iPod touch (5th generation), iOS 8.1
It means that the app is a 'universal' app i.e. it's designed for both iPads and the smaller iPhone/iPod Touch sized screens (it has processing within it that detects the device that it's installed on and makes best use of the device's screen e.g. if used on an iPad it will make use of the larger screen)