The major benefit of the Incident Command System (ICS) is that it can expand in a logical manner from an initial isolated incident into a major widespread disaster with the least disruption of systems and resources. Here is the basic structure of the ICS.
The Command Staff consists of the Information Officer, Safety Officer, and Liaison Officer. They report directly to the Incident Commander. They may have a deputy (must meet same ICS requirements as the "Officer") as needed. They may also have an assistant or assistants as needed (However, as an 'assistant' they do not need the same ICS requirements as the "Officer" or "Deputy"). The Command Staff reports directly to the Incident Commander.
The General Staff is the group of incident management personnel charged with mitigating the incident. They report directly to the Incident Commander. They may each have a deputy (who must meet same ICS requirements as the "Section Chief")
Deputy (whatever position) must meet the certifications & qualifications of the 'full' position as the Deputy might have to take over for the position. Assistant (to whatever position) is just that, 'a helper', and therefore do not need to meet the certifications & qualifications of the position they are assisting.