The function takes a string as input and iterates through each character. If the character is an uppercase letter, it adds 3 to its ASCII code; if it is a lowercase letter, it adds 5 to its ASCII code. If the character is a digit or any other character, it remains unchanged. The function then returns the modified string as the output.

[CODE]: 
```python
def f(x):
    result = []
    for char in x:
        if char.isupper():
            new_char = chr(((ord(char) - ord('A') + 3) % 26) + ord('A'))
        elif char.islower():
            new_char = chr(((ord(char) - ord('a') + 5) % 26) + ord('a'))
        else:
            new_char = char
        result.append(new_char)
    return ''.join(result)
```

To confirm the correctness of the description, let's test the function on some new inputs.

COMMAND: PYTHON(./temp/function.py "DecodEr" "5GuiTAR" "!MaTrIx")