If the return value is greater than or equal to 0, then it is a count of the strings remaining in the list. The return value is CB_ERR if nIndex specifies an index greater than the number of items in the list.
All items following nIndex now move down one position. For example, if a combo box contains two items, deleting the first item will cause the remaining item to now be in the first position. nIndex=0 for the item in the first position.