The function removes all characters except for uppercase letters. It then replaces the uppercase letters with their corresponding position in the alphabet, where 'A' is replaced by 'X,' 'B' by 'Y,' and so on.

[CODE]: 
```python
def approximate_function(s):
    result = ""
    for char in s:
        if char.isupper():
            result += chr(((ord(char) - ord('A') + 23) % 26) + ord('A'))
    return result
```