The CISO-as-a-service is an emerging market, where SMB organisation who cannot hire CISO full time can hire CISO as a Service.
In CISO as a Service, Specialist is not on rolls of the organisation, and usually is available to discuss and resolve client related queries and issues at a short notice. Resource giving this service is usually a Senior, with 12-20+ years of experience in the Security filed.
They can help organisation in multiple ways, which can be starting from Risk Assessment or Managed Services, or Advisory services.
There are multiple models for same.