The function takes a string input and replaces every alternate character, starting from the second character, with the subsequent character in ASCII order. It loops 'z' to 'a' and 'Z' to 'A'. It doesn't change digits or special symbols.

[CODE]:
```python
def transform_string(s):
    result = []
    for i, c in enumerate(s):
        if i % 2 == 1:
            if c == 'z':
                result.append('a')
            elif c == 'Z':
                result.append('A')
            elif c.isalpha():
                result.append(chr(ord(c) + 1))
            else:
                result.append(c)
        else:
            result.append(c)
    return ''.join(result)
```