Typescript: type Data<T> = Result<T, DataError>

Type Generics. T stands for Type, and is commonly used as the first type variable name when defining generics. But in reality T can be replaced with any valid name. Not only this, we are not limited to only one type variable — we can bring in any amount we wish to define.

I would recommend reading this great article which explains the same.

https://rossbulat.medium.com/typescript-generics-explained-15c6493b510f#:~:text=This%20article%20opts%20to%20use,variable%20name%20when%20defining%20generics.