The function replaces all vowels ('a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U') in the input string with the uppercase letter 'L'.

[CODE]:
```python
def f(x):
    vowels = 'aeiouAEIOU'
    result = ""

    for char in x:
        if char in vowels:
            result += 'L'
        else:
            result += char

    return result
```
This code approximates the behavior of the original function from the provided test outputs.