What does "archival quality" mean in the context of journal aims and scope? What does "archival quality" mean in this context? Is it just a posh way of saying they only want a paper if it describes an important breakthrough?


  IEEE Transactions on Evolutionary Computation publishes archival
  quality original papers in evolutionary computation and related areas
  including nature-inspired algorithms, population-based methods, and
  optimization where selection and variation are integral, and hybrid
  systems where these paradigms are combined. Purely theoretical papers
  are considered as are application papers that provide general insights
  into these areas of computation.


Note: I think this may be a duplicate of What does archival contribution in journals mean. However, even if it gets closed as a duplicate, it will help others searching for the information. And it would be nice to have a more extensive answer, either here or on the other question.
Generally "archival quality" implies some level of "completeness" and comprehensiveness (to the degree that any work of science and engineering can really be "complete.") In many fields, early work can be published in some conferences, but journal papers that are expected to be "archival quality" are supposed to be more mature. 

To quote from this Guidelines for Expanding Conference Papers for Submission to the IEEE Sensors Journal:


  Conference papers are usually short, and deal with very recent research results or new ideas illustrated by preliminary experiments. An archival journal paper, on the contrary, contains an overview of the state-of-the-art (with references to other’s and one’s own past work), a problem statement, the concept of a solution, a discussion of the theoretical
  background, experimental set-up, sound measurement data, an evaluation of the results, discussion and conclusions – a much more complete presentation of the overall project and its results.