VS code snippets - what is the required number in front of a choice list?

https://code.visualstudio.com/docs/editor/userdefinedsnippets#_choice

${1|one,two,three|}

So I'm new to snippets, and I noticed the number in front of a list of completion choices is required to show the choice menu correctly (if omitted, it will populate the choice list as a literal string - https://github.com/infosec-intern/textmate-yara/pull/29).

What does this number do, and why is it needed ? (the documentation doesn't explain it)

is it for the default selection ?

Thanks,