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A size declarator is a number used in the declaration of an array to specify the number of elements that the array can hold. It determines the total memory allocation for the array and represents the amount of storage space required to store all the elements in the array.

A subscript, also known as an index, is an integer value used to access an individual element within an array. By providing a subscript in square brackets [ ], you can target a specific element in the array, which is helpful in assigning values to array elements or accessing an element's value.

In an array declaration, a size declarator is used within square brackets [ ] right after the array variable name and data type to allocate the memory required for the specified number of elements in the array. For example, in C++: ```cpp int numbers[5]; // Declare an integer array named 'numbers' with a size of 5 elements ``` In this declaration, '5' is the size declarator, which indicates that the 'numbers' array can hold up to 5 integer elements.

Subscripts are used to access individual elements in an array by using their index (position) in the array. The index starts from zero, so the first element of an array is at index 0, the second element is at index 1, and so on. For example, to assign values to the 'numbers' array we declared earlier, we can use subscripts as follows in C++: ```cpp numbers[0] = 1; // Assign the value 1 to the first element (index 0) of the 'numbers' array numbers[1] = 2; // Assign the value 2 to the second element (index 1) of the 'numbers' array ``` In this example, '0' and '1' are subscripts, which represent the indexes of the two first elements in the 'numbers' array. In conclusion, size declarators are used to define the size of an array at the time of declaration, while subscripts (indexes) are used to access individual elements within the array.