HSI
From OHRRPGCE-Wiki
HSI stands for HamsterSpeak Include file. An HSI file is just a plain text file, just like a script file.
You can use whatever extension you want for your script files. But ".hsi" is mainly used just for the file created for your game which has a list of constants for all the names of Heroes, Maps, Items, and various other things in your game, which you can use in your scripts.
You can create this file by going to the Script Management menu and selecting Export Script Names (.HSI), but you never need to do so, because it happens automatically whenever you import your scripts. Also, you don't need to include the hsi file in your scripts, because it's done automatically.