Stan,

That is referring to the RAW engine used to translate the RAW data into an image you see on screen. The code to do this is constantly being improved, which should mean that your images will look even better when decoded with a newer version than they were with the old one. However we can’t have our images suddenly changing under our feet, so when a new process is available, Aperture offers you the option to reprocess the image — knowing that your image may change how it looks. For this reason when you do it, you have the option to create a new version and update that, so you have your previous version available still as well.

This only applies to RAW files.

Personally I only go back and reprocess old files if I’m going to use it again. If it’s one I’ve already adjusted, I will make a new version first, and will often throw away previous adjustments and start over. Sometimes I go back to the one I already had done, sometimes I like the new version better. Just depends on how it turns out.

John,

You would only see this if you had an Aperture 1 or 2 library that had been upgraded, and then only on those older photos.