I was going through solidity documentation and found this paragraph inside "Function Types" section https://docs.soliditylang.org/en/v0.8.21/types.html#user-defined-value-types
A function of an internal type can be assigned to a variable of an internal function type regardless of where it is defined. This includes private, internal and public functions of both contracts and libraries as well as free functions. External function types, on the other hand, are only compatible with public and external contract functions.
What is a internal function type variable, does it even exists. And if not, what does this para mean?