Based on the experiments, it seems like the function implemented by f(x) takes a string input and performs the following operations:
1. If the input string contains the substring "abc", it replaces every occurrence of the letter 'a' with 'b' and appends the letter 'X' to the end.
2. If the input string contains the substring "xyz", it replaces every occurrence of the letter 'y' with 'o'.
3. If none of the above conditions are met, the function simply returns the input string as is.

Here is a Python code approximation of the function:

[DESCRIPTION]:
The function looks for specific substrings in the input string and performs certain operations based on the presence of those substrings.

[CODE]:
```python
def f(x):
    if "abc" in x:
        return x.replace('a', 'b') + 'X'
    elif "xyz" in x:
        return x.replace('y', 'o')
    else:
        return x
```

This code checks if the input string contains "abc" and "xyz" substrings using the `in` operator. Based on the conditions, appropriate string manipulation operations are performed using the `replace()` method, and the modified string is returned. If none of the conditions are met, the original input string is returned unchanged.