The default value is the value used to pad the array when the array expands. For example, if you set the array index to a value beyond the defined portion of the array and enter a value in an element greater than one row past the defined portion of the array, the elements between the previous end of the array and the element you added are set to the default value.
You can use the Array Size function to determine the size of the array. You can index beyond the bounds of an array inadvertently by indexing an array past the last element using a While Loop, by supplying too large a value to the index input of an Index Array function, or by supplying an empty array to an Index Array function.