You will find in every app review and How To guide will first explain if the app is a stand alone or sharing application.

You should think through what you would like your iPhone to be to you prior to making application purchases. When we say ‘stand alone’ or ‘sharing’ we are talking about the data you input into the aftermarket application you spent your money on. 

Will you need to share information with others at work, in your household, in meetings? If you are creating or editing a document (text file, database, spreadsheet, calendar, ToDos, etc…) is it helpful to be able to share that item with others for their use and review.