Experts in a Field
PIs who have been specially trained (usually in law enforcement or in the military) and/or have gone to school for a specific subject matter, will market themselves as experts in a specific field. This field may be one of the many forensic sciences or cyber analysis fields.
More often than not, one who specializes in one of the specific fields will only take cases that specifically related only to that field. Such fields include: Blood Stain Patterns, Tire Tread and Tracks, Took Marks, Cyber, Ballistics, Human Trafficking, Crime Scene Investigations, Digital Photo Imaging, Forensic Anthropology, Forensic Taphonomy, Forensic Entomology, Forensic Toxicology, Vehicular Accident Reconstruction, Fire and Explosions, Documents Fraud, and Firearms.
More often than not, you will not be able to hire one directly (they are usually even difficult to find), but you will need to acquire their services through an attorney or another General Private Investigator.