The function takes a string input and replaces lowercase alphabetic characters with uppercase characters. All other characters are replaced with 'X'.

[CODE]:
```python
def f(x):
    output = ""
    for c in x:
        if c.isalpha() and c.islower():
            output += c.upper()
        else:
            output += "X"
    return output
```

This code will transform the input string by replacing lowercase alphabetic characters with their uppercase counterparts and replacing all other characters with 'X'.