Custom spell loading screens

Hawdey!
While browsing today's changelog for our beloved realm Frostwolf, i found this rather interesting line in the General Fixes part.

This line was saying "Added support for custom spell loading screens, also fixes default loading screen showing in some cases", reading this made me understand that there has been a new feature implemented that allow us to change the Loading screens, but i'm wondering how is this possible.

And also, when it says "custom spell loading screen", what is the Spell referring to and why?

This change is for the Isle of Thunder, where depending on your faction and progress server teleport spells to the isle show different loading screens to the client.
This isn't something a player can directly influence.