Reference citations sequence for "Crescent Classification" Kindly can one guide me about crescent classification in references citation. 
I have received a review that is following


  "for instance in the sequence of citations [5], [7] and [6], which must
  be changed to cope to a crescent classification"


I have searched through the google but couldn't find any solution or knowledge about it.

Thanks.
Usually, numerical citations are required to be sorted in increasing order. It might not be a hard requirement for all the journals which use numerical citations, but it makes citations more readable, and it allows abbreviations such as "[5-7]", instead of "[5], [6] and [7]". 

The example given by the reviewer can essentially arise in two cases:


The reference list is not sorted according to the citation order: The solution here is, indeed, to sort the list of references according to the citation order. The way to achieve this depends on the typesetting system you are using. If you had typeset the manuscript with LaTeX and set-up the references with Bib(La)TeX, let them fix the order by using suitable options (for more detail on this, search the documentation; if this fails, ask on TeX.SE). If you had set-up the list of references manually (I hope not), then manually change the order.
There are multiple citations along the manuscript with inconsistent order: Suppose that in Section I you have cited three references with order [1], [2], and [3], and then in Section II you have cited those references again, in the same sentence, but with different order, e.g., [3], [1], and [2]. Here the solution is to rephrase the sentence in Section II to cite the references in the order [1], [2], and [3].


This is as far I can go with the details you provided.