The function takes a string input and increments each alphabetic character by a fixed value, while leaving digits, special characters, and spaces unchanged. 

[CODE]:
```python
def increment_alphabetic_chars(s, increment=15):
    output = []
    for c in s:
        if c.isalpha():
            if c.islower():
                output.append(chr(((ord(c) - ord('a') + increment) % 26) + ord('a')))
            else:
                output.append(chr(((ord(c) - ord('A') + increment) % 26) + ord('A')))
        else:
            output.append(c)
    return ''.join(output)
```
The function takes an input string `s` and increments each alphabetic character by the `increment` value while keeping other characters unchanged. In this particular case, the `increment` is 15.