You're seeing just the callback handle_validate, go back to Cqrs.ValueObject module for more information.
Link to this callback

handle_validate(arg1, keyword)

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handle_validate(Ecto.Changeset.t(), keyword()) :: Ecto.Changeset.t()

Allows one to define any custom data validation aside from casting and requiring fields.

This callback is optional.

Invoked when the new() or new!() function is called.