The so called pre-scripts are a various set of component specific scripts to be deployed before the main upgrade. Each of these scripts must be evaluated one by one to verify if they are relevant for the current customer before they can be deployed into the database. Because of this, they must be handled manually and cannot be part of the automatic installation process. The mandatory Pre scripts might only come in question when going into IFS Cloud from earlier releases, but there could be optional scripts to consider going between two IFS Cloud versions.
The pre-scripts are described here.