The function f(x) returns the input string unchanged if it consists only of numeric characters. Otherwise, it performs a transformation on the non-numeric characters.

[CODE]:
```python
def f(x):
    if x.isdigit():
        return x
    else:
        transformed_string = ""
        for char in x:
            if char.isalpha():
                transformed_char = chr(ord(char) + 2)
                transformed_string += transformed_char
            else:
                transformed_string += char
        return transformed_string
```

Please note that this code is an approximation and may not capture all the nuances of the function's behavior.