When to use static keyword before global variables?

Question

Can someone explain when you\'re supposed to use the static keyword before global variables or constants defined in header files?

For example, lets say I have a head

Answer 1

The static keyword is used in C to restrict the visibility of a function or variable to its translation unit. Translation unit is the ultimate input to a C compiler from which an object file is generated.

Check this: Linkage | Translation unit