Semantics of fields and exclude

[pm13]

I tried to generalized fields and exclude options from ModelForm to be
useful also for Form (for example for including and excluding fields
from possible superclasses or for ordering). And I have two questions
about this:

1. Fields which are not in models:

This example (django.contrib.auth.forms):

class UserCreationForm(forms.ModelForm):
"""
A form that creates a user, with no privileges, from the given
username and password.
"""
username = forms.RegexField(label=_("Username"), max_length=30,
regex=r'^\w+$',
help_text = _("Required. 30 characters or fewer. Alphanumeric
characters only (letters, digits and underscores)."),
error_message = _("This value must contain only letters,
numbers and underscores."))
password1 = forms.CharField(label=_("Password"),
widget=forms.PasswordInput)
password2 = forms.CharField(label=_("Password confirmation"),
widget=forms.PasswordInput)

class Meta:
model = User
fields = ("username",)

Option fields say - get everything from model, filter fields and add
everything from form. I think it would be better: get everything from
model and form and then filter fields - for example, what if there is
a useless field in a superclass?

class Meta:
model = User
fields = ("username", "password1", "password2")

2. Fields and exclude in the same form:

This example (modeltests.modelforms.models):

class Category(models.Model):
name = models.CharField(max_length=20)
slug = models.SlugField(max_length=20)
url = models.CharField('The URL', max_length=40)

class CategoryForm(ModelForm):

class Meta:
model = Category
fields = ['name', 'url']
exclude = ['url']

It could mean two things (without reading of code I cannot say what is
right):
a) exclude 'url' and then include 'name' and 'url' - result is 'name'
and 'url'.
b) include 'name' and 'url' and then exclude 'url' - result is 'name'.

Option B is right, but it can be more simple written so:

class CategoryForm(ModelForm):

class Meta:
model = Category
fields = ['name']

Why there are two different ways? Is it not better to forbid to use
fields and exclude at once?