What is font ligature stylistic sets?

Question

I was trying to setup Fira Code font on vscode, which supports ligatures.

I came to this page, which states,

Stylistic sets Since version 1.40, you can define stylistic sets in settings.json:

"editor.fontLigatures": "'calt', 'ss01', 'ss02', 'ss03', 'ss04', 'ss05', 'ss06', 'zero', 'onum'"

I tired "editor.fontLigatures": true and ligature works.

But I wonder now, what difference stylistic sets make?