Only as formal parameters in a function definition. Otherwise, they're not. I'm assuming we're talking about parameters for the answer to your next question.
when is the formal (char** val) used
I'll use the array notation when I'm absolutely sure that the function is intended to work with an array or simulated array. I prefer to use pointer notation when the object is known to not be an array or whether it's an array is unknown.
so if i get you, they are all array notation but char** val is implicite and char* val is explicite
because i just noticed that using int main (int argv, char** argc) worked the same way as int main (int argv, char* argc)
they are all array notation but char** val is implicite and char* val is explicite
It's the other way around. They are all pointers, and the  option is syntactic sugar to highlight the intention that the parameter is an array. But also note that this equivalence only applies to the first dimension of a the array notation. All subsequent dimensions require a size and represent array types. Consider this:
foo() must be declared in one of two ways:
void foo(int a); /* First dimension decays into a pointer */
void foo(int (*a)); /* Explicitly stating the first dimension as a pointer */
It's a common misconception that the double pointer type corresponds to a 2D array, which is false:
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