Space for Null character in c strings

When is it necessary to explicitly provide space for a NULL character in C strings.
For eg;
This works without any error although I haven't declared str to be 7 characters long,i.e for the characters of string plus NULL character.

#include<stdio.h>

int main(){
    char str[6] = "string";
    printf("%s", str);
    return 0;
}

Though in this question https://stackoverflow.com/a/7652089 the user says
"This is useful if you need to modify the string later on, but know that it will not exceed 40 characters (or 39 characters followed by a null terminator, depending on context)."

What does it mean by "depending on context" ?