The function assigns a numerical value to each input string based on some criteria. In this particular case, it seems that the function assigns the value "one" to strings that have the letter "a" in them, and the value "four" to strings that have the letter "b" in them. Otherwise, it assigns the value "one".

[CODE]:
```python
def f(x):
    if 'a' in x:
        return 'one'
    elif 'b' in x:
        return 'four'
    else:
        return 'one'
```

Please keep in mind that this is just an approximation based on the limited experiments we ran. It's possible that the function behaves differently for other input strings.