The function takes an input string and performs a substitution cipher, where each character in the input string is replaced with the corresponding character in a fixed string "wxvmrk". Additionally, the function appends the string "B" to the end of the input string.

[CODE]:
```python
def f(x):
    fixed_string = "wxvmrk"
    output = ''
    for char in x:
        output += fixed_string[ord(char) - ord('a')]
    output += 'B'
    return output
```
In the code above, the `f(x)` function performs the substitution by iterating over each character of the input string and replacing it with the corresponding character in `fixed_string`. The `ord` function is used to convert the character to its ASCII value, then subtracting the ASCII value of 'a' to get the index in `fixed_string`. Finally, the function appends 'B' to the end of the output string before returning it.