This is the row index in the table of Anti-Virus scanning engines active on this gateway, since a Check Point AV deployment can run more than one engine, for example a signature-based engine alongside a heuristic one. This index is what an admin uses to correlate the name, version, and date columns for each individual engine instance when more than one AV engine is loaded, rather than conflating stats from different engines. For example, index 1 tied to a signature-based engine and index 2 tied to a heuristic engine on the same gateway.