Why the GHRSST L3C from AVHRR named subskin?

Hi,
As I know the definition of SST_subskin is from microwave satellite data. Since GHRSST L3C is from AVHRR only which is IR data, why can we call it SST_subskin?
Thanks in advance!
As I know the definition of SST_subskin is from microwave satellite data. Since GHRSST L3C is from AVHRR only which is IR data, why can we call it SST_subskin?
Thanks in advance!