Hemagglutanin & Neurominadase are surface antigens. The number ‘1’ refers to the specific sub-type of those antigens. The sub-types differ in their basic structure.
There are a number of different viruses with variations of this same basic name, such as H2N2, H3N2, H5N1 and so on..
It’s pretty meaningless unless you look at them under a microscope!