What's the difference between OneToOne and Subclassing a model in Django

For example:

class Subdomain(Site):
    #fields 开发者_如何转开发here

and

class Subdomain(models.Model):
    site = models.OneToOne(Site)
    #fields here

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Models with a OneToOne have an independent PK; submodels always use the PK of their supermodel.