ITNinja

When an MSI entry point is triggered all files marked in the MSI as keypaths are verified to exist before execution. Each component in a feature contains one or more elements, one of which is marked as a keypath. When discovered to be missing, an installation is triggered at the feature level (for repackaged applications, this is often the entire package). Checks to verify if a file is the correct version, or if the file is corrupt is possible, but is not the default behavior. Such a check must be specified when the MSI is authored as to perform such a check of all files would be an intensive process that would affect system performance.

Some more information and for difference between self heal and repair see this link:

http://msiworld.blogspot.com.au/2008/10/difference-between-self-heal-and-repair.html