You're seeing just the function change_default_attribute, go back to Ash.Changeset module for more information.
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change_default_attribute(changeset, attribute, value)

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change_default_attribute(t(), atom(), any()) :: t()

The same as change_attribute, but annotates that the attribute is currently holding a default value.

This information can be used in changes to see if a value was explicitly set or if it was set by being the default. Additionally, this is used in upsert actions to not overwrite existing values with the default