What merits acceptance for rapid publication in letter format? Ive recently been asked to review for a letter format journal (articles less than 4 pages) for rapid publication. The same standard criteria of original content, novelty and relevant references as for other journal publications are required. Looking to the referee guide of the journal, it is stated that 


  When reviewing, reviewers should ask themselves 'Is this paper suitable for rapid publication?'


I am unable to find any discussion or guideline that clearly state what merits acceptance for rapid publication compared to the normal peer-review process for journal article submissions. Publishing as a IEEE letter vs article gives some input from the guidelines of IEEE JMEMS, where the author should argue why it is eligible for rapid publishing, although, Im not sure if this guideline is general for all journals. 

I can understand that rapid publishing has benefits for authors and can be required by publishers due to competition. But I dont see what I, as a reviewer, am supposed to look for.

Since the letter format is short, not all aspects can be covered in the submission. Since the decision is binary (accept/reject), questions related to the submission cannot be answered or clarified by the authors through the review process. The situation arises that the submission can be original and novel, but may lack investigations or results. Should I still accept?
From my understanding of the term, it seems to me that your job is to evaluate two things:


Is the manuscript in "good enough" shape to publish without a proper, more rigorous peer-review process? As you say, there's no opportunity for a back-and-forth with the authors, although presumably they would have an opportunity to incorporate your feedback. So you'd need to be confident that the manuscript will be publishable with just one round of revisions.
Is there a clear, urgent need for publication that warrants circumventing the traditional peer-review process? I think the editor is looking for your recommendation, given your knowledge of the field and your assessment of how timely and significant the contribution is, as to whether to publish the manuscript as a letter, rather than as a regular research article. I imagine the authors won't make an argument for rapid publication specifically in the manuscript (due to lack of space), but perhaps in an accompanying cover letter or something similar (as indicated in this answer).


Regarding your specific question about a submission that is "original and novel, but may lack investigations or results," I think it is your job to judge:


whether publishing the manuscript essentially "as is," without such investigations, will sufficiently spur further advances in the field such that it's worth not waiting for those results; and
whether there is significant risk of similar, cutting edge work being published elsewhere earlier if this manuscript is subjected to a full peer-review, that would undercut both these authors and the journal you're reviewing for.