How do Build and RE-Build differ?

Build creates directories and pages in your server as directed by your Project. Build randomizes ALL Spintax it finds.

RE-Build performs the same process as Build. But, Spintax is randomized ONLY in added or modified pages. All other Spintax is "randomized" in exactly the same way it was in the previous Build.

This allows building a modified Project multiple times without Google seeing ALL pages as new. Only added or modified pages will be seen as new. This essentially allows manual "Drip Feed".

Additional info can be found in the Getting Started Video Build and RE-Build.