The tables that are used to rank the mutations were created through literature research.
An overview of the papers they are derived from can be found below. The corresponding papers for every single mutation can be found in the source column of the table.
Those are mutations that have been identified previously in the literature as changing the potential of the virus.
Kirkpatrick, Ericka & Henry, Carole & McMahon, Meagan & Jiang, Kaijun & Strohmeier, Shirin & Bakel, Harm & Wilson, Patrick & Krammer, Florian. (2020). Characterization of novel cross-reactive influenza B virus hemagglutinin head specific antibodies that lack hemagglutinin inhibition activity. Journal of virology. 94. 10.1128/JVI.01185-20.
Liu, Yi & Tan, Hyon-Xhi & Koutsakos, Marios & Jegaskanda, Sinthujan & Esterbauer, Robyn & Tilmanis, Danielle & Aban, Malet & Kedzierska, Katherine & Hurt, Aeron & Kent, Stephen & Wheatley, Adam. (2019). Cross-lineage protection by human antibodies binding the influenza B hemagglutinin. Nature Communications. 10. 10.1038/s41467-018-08165-y.
Brown, Sook & Tseng, Yeu-Yang & Aziz, Ammar & Baz, Mariana & Barr, Ian. (2022). Characterization of influenza B viruses with reduced susceptibility to influenza neuraminidase inhibitors. Antiviral Research. 200. 105280. 10.1016/j.antiviral.2022.105280.
Zaraket, Hassan & Hurt, Aeron & Clinch, Barry & Barr, Ian & Lee, Nelson. (2021). Burden of influenza B virus infection and considerations for clinical management. Antiviral Research. 185. 104970. 10.1016/j.antiviral.2020.104970.
Those are important regions of the gene like for example Antigenic sites or Receptor-binding sites.
Qinghua Wang et al. “Crystal Structure of Unliganded Influenza B Virus Hemagglutinin”. In: Journal of virology 82 (Jan. 2008), pp. 3011–20. doi: 10.1128/JVI.02477-07.
Qinghua Wang et al. “Structural basis for receptor specificity of influenza B virus hemagglutinin”. In: Proceedings of the National Academy of Sciences of the United States of America 104 (Nov. 2007), pp. 16874–9. doi: 10.1073/pnas.0708363104.