"The procedural validity of texts submitted to a referendum Questions submitted to a referendum must respect: - unity of form: the same question must not combine a specifically-worded draft amendment with a generally-worded proposal or a question of principle; - unity of content: except in the case of total revision of a text (Constitution, law), there must be an intrinsic connection between the various parts of each question put to the vote, in order to guarantee the free suffrage of the voter, who must not be called to accept or refuse as a whole provisions without an intrinsic link; the revision of several chapters of a text at the same time is equivalent to a total revision; - unity of hierarchical level: it is desirable that the same question should not simultaneously apply to legislation of different hierarchical levels. "
CoE (Venice Commission): Code of Good Practice on Referendums, para. III.2

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Code of Good Practice on Referendums