The function takes a string as input and calculates the sum of character positions in the alphabet (1-based) for each character in the input string. It then returns the sum as a string with the suffix 'D'.

[CODE]:
```python
def approximated_function(input_string):
    alphabet = 'abcdefghijklmnopqrstuvwxyz'
    total = sum([alphabet.index(ch.lower()) + 1 for ch in input_string if ch.lower() in alphabet])
    return f"{total}D"
```