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

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change_new_attribute_lazy(t(), atom(), (() -> any())) :: t()

Change an attribute if is not currently being changed, by calling the provided function

Use this if you want to only perform some expensive calculation for an attribute value only if there isn't already a change for that attribute