The initintro.sqs file is loaded automatically by the Operation FlashPoint engine when the mission's intro begins. The file resides in the root of the mission directory.
The initintro.sqs file is a good place to load scripts, and variables before the mission's intro begins.
Note: In editor, initintro.sqs is loaded both in Intro and Outro sections, but it's editor bug. Do not use this script to execute outro variables!