Generally, a publication is an unethical dual publication if
- the editor didn't know about the previous publication (i.e. the authors did not disclose it), or
- the author didn't cite the previous publication (and policy requires them to do so), or
- the authors have violated the "dual publication" or "extended paper" policy of the journal, which may have its own specific requirements. (For example, many, but not all, journals in CS and EE have a policy that conference papers republished as journal papers must contain 25% new material, a determination that is made by the editors based on the reviewers' assessments.)
In your example, the authors do cite the previous publication in their reference list. There is no way for you, the reader, to know whether the editor was aware of the previous publication during the review process, but it seems likely (since the authors cited their previous work.) If you're concerned that they didn't know about the previous publication, you can always send the editors an email and ask them.
The policies of IEEE, the IEEE Communication Society, and JSAC, all permit conference papers to be re-published in journal form, as long as it's cited and properly disclosed to the editor at submission.
The document you've cited from the IEEE is about using CrossCheck, a plagiarism detection tool that finds "similarity" between a submitted document and previously published material. It says:
Interpreting Matching Percentages of Individual Sources
It may seem that any source of matching text should be a concern, but in fact many matching sources are likely to not be the result of plagiarism. For example:
>25% match - This level of similarity from a single source should raise serious concerns about inappropriate reuse, and should be checked very carefully
which is probably why you believe that IEEE punishes authors "who publish material which uses more than 25% of the content from another publication." That document then it goes on to say
Factors to Keep in Mind when Reviewing Individual Sources
Is the similarity to the authors’ own work?
This can often be the case. Authors build upon their own previously published work, and will often reuse portions of text. While this would not be considered plagiarism, it may still indicate a potential problem if the reuse of previously published content is not cited properly.
The plagiarism discussion with the ">25%" guideline is not about authors who reuse their own material. A separate "self-plagiarism" policy applies to them. The policy on authors republishing their own work (in identical or modified form) is called the "dual publication policy." The IEEE Communication Society's (JSAC is a ComSoc journal) dual publication policy is:
The guidelines recognize that it is common in technical publishing for material to be presented at various stages of its evolution. As one example, this can take the form of publishing early ideas in a workshop, more developed work in a conference and fully developed contributions as journal or transactions papers. This publication process is an important means of scientific communication. The editor of a publication may choose to re-publish existing material for a variety of reasons, including promoting wider distribution and serving readers by aggregating special material in a single publication. This practice continues to be recognized and accepted by the IEEE. At the same time, the IEEE requires that this evolutionary process be fully referenced by the author.
Authors submitting manuscripts must disclose whether there are prior publications, e.g. conference papers, by the authors that are similar, whether published or submitted. They must also include information that very clearly states how the new submission differs from the previously published work(s). Such papers should be cited in the submitted manuscript.
In other words, they permit re-publishing as long as the editor agrees and:
- On submission, the authors disclose that the work is previously published, and state the specific differences between the new submission and the previous publication.
- The previous publication is cited in the new submission.
This is consistent with the general IEEE policy on author originality.
The journal's specific policy is also consistent with the IEEE and IEEE ComSoc policies:
With respect to authors' previously published or to-be-published work, material which has been previously copyrighted, published, or accepted for publication will not in general be considered for publication in J-SAC. Exceptions to this rule include an author's prior publications that have had limited distribution, have been printed in abstract form only, or have appeared in conference records or digests. All papers are considered on the basis of their individual merit alone, and the fact that a paper may have been accepted for presentation at a conference does not ensure its acceptance for publication in J-SAC. In many cases, a conference paper must be substantially revised to meet the technical standards maintained by J-SAC. A manuscript identical to or largely based upon a conference paper must be so identified. Please refer to IEEE's policy on originality of content for more detailed information on the use of previously-published work as the basis for a J-SAC submission.
- Work that has only appeared in conference proceedings may be considered for publication.
- Authors must disclose at paper submission time if they are submitting a paper that is "largely identical" to an already published conference paper.