I can request to have my PhD certificate name a more specific research area than "PhD in Computer Science". What should I have it say?
Many employers and recruitment agencies use either automated searches or low skill human readers to select resumes that meet specific requirements. Only those resumes will be viewed by a human with knowledge of the job etc.

If you pick an unusual subject designation for your PhD you risk your resume not being selected for jobs that require a PhD in computer science. It may never be seen by anyone who understands that "Applied Machine Learning" is just as appropriate for that job as "Computer Science".

If your resume passes the filter it will be read by someone who can evaluate your specialization from the text and the publication list.