I have an app that I developed with Xcode 3 and recently started editing with Xcode 4. In the target summary I have the iOS application target form with fields: identifier, version, build, devices, and deployment target. The version field is blank and the build field is 3.4.0 (which matches the version of the app from when I was still editing with Xcode 3).

My questions are:

1. What is the difference between the version and build fields?

2. Why was the version field blank after I upgraded to Xcode 4?