I wrote about this on my blog a few months ago, titled Difference between CFBundleVersion and CFBundleShortVersionString. It's simple, but confusing if you haven't been introduced to it:
Click on the Apple icon in the Mac menu bar and choose About This Mac. Click "Version 10.xxx". You should see something like this:
Version 10.10.3 (14D113c)
CFBundleShortVersionString (10.10.3) is the version number and is the version number that is public and marketed to users. CFBundleVersion (14D113c) is the build number.
Naturally, you should increase the CFBundleVersion (build number) with every build, example for testing - TestFlight, HockeyApp.
You also (and only) increase the CFBundleShortVersionString (version number) when it's a new release, e.g. when Apple starts beta for 10.10.3, all builds should have version=10.10.3 instead of the previous 10.10.2 and all builds will have different build numbers.
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