Been a long time since I studied criminal law, but my memory is that detainment only requires reasonable suspicion that a crime is being or has been committed, though it can be done for the officers safety in the course of an investigation, reasonable suspicion still required, but the detained person may not be the target if its a safety concern. It's ultimately for the purpose of investigation.

Arrest requires probable cause that a crime has been committed, and the person arrested is the accused.