What is a subgenomic promotor?
A subgenomic promoter is a promoter added to a virus for a specific heterologous gene, resulting in the formation of mRNA for that gene alone. Many positive-sense RNA viruses produce these subgenomic mRNAs (sgRNA) as one of the common infection techniques used by these viruses and generally transcribe late viral genes. From doi:10.1128/jvi.74.13.5988-5996.2000
So it seems to be an added promoter that helps express certain genes. Often useful in viral engineering.