Purpose of "extern" keyword in a function declaration

What is the purpose of "extern" keyword in a function declaration?

- The only other storage class possible for a function is static, which must be specified explicitly.

- It cannot be applied to a block scope function declaration and results in internal linkage.

- The keyword 'exter' indicates the actual storage of the variable is visible, or body of a function is defined elsewhere, commonly in a separate source code.

- This extends the scope of the variables or functions to be used across the file scope.

Example:

extern int number;