Authorship or a co-author means a person who has made a significant contribution to manuscript and who shares responsibilities and accountability of the results.  If a manuscript is written by more than one author, you all choose one person to be the corresponding author. This person will handle all correspondence about the manuscript and sign the publishing agreement on behalf of all the authors. If you are a co-author, this means that

1. You have made a significant contribution to the research, whether it is in the concept or design, acquisition of data, analysis and interpretation, or in all these areas.
2. You have drafted, written, or revised the article.
3. You have reviewed and agreed on the final version of the article before submission.
4. You have agreed on the journal to which your manuscript will be submitted.
5. You are aware that you are taking responsibility and accountability for the content of the manuscript.
6. You are aware that the corresponding author will be acting on your behalf in any communication about the article, through submission, peer review, production, and after publication.
7. In line with standard publishing ethics, if your article is found to be unsafe, have errors, in some way fraudulent, or in breach of the publishing agreement, that responsibility is shared by all named co-authors.