Submission of a manuscript signifies that it has been neither copyrighted nor published, submitted, or accepted for publication elsewhere. IEEE policy requires that when using their own previously published or submitted material as a basis for a new submission authors are expected to "cite the previous work(s) and very clearly indicate how the new submission differs from the previously published work(s)." [IEEE Publications Operations Manual, Section 8.2.4F.]
If a submitted manuscript has been published or has been accepted for publication in the Proceedings of an IEEE conference, it may be considered for publication in the Transactions of Automatic Control if evidence is provided that it adds value relative to its conference version (for example, it contains detailed proofs omitted from the conference version, or additional numerical results). This ensures consistency with the policy stated in the previous paragraph.
If, at its time of submission, a manuscript has also been
submitted for publication in the Proceedings of an IEEE conference, it will
be considered for publication in the Transactions of Automatic Control
with the understanding that, should it be found publishable in both venues,
evidence will be provided that its final version adds value relative to
its conference version, as explained in the previous paragraph.