ItemType Generic Utility Type
This is to describe the situation where one might want to use the type of an array indirectly.
The ItemType generic is sugar over an existing capability in TypeScript to access the type information of the Array generic and make accessing that type more legible to the reader. But a few people replied with "huh, I don't think you want to do that", and to their point, I agree... mostly.
It's almost always better to declare a variable to be of the Array's type directly.