What does `L:` mean in `"injectionSelector": "L:markup.fenced_code.block.markdown"`?

At https://github.com/mjbvz/vscode-fenced-code-block-grammar-injection-example/blob/master/syntaxes/codeblock.json

The extra grammar is injected into the scope markup.fenced_code.block.markdown

However L:markup.fenced_code.block.markdown is used. What does L: mean in this case? Is it a special syntax for scope selector? I cannot find any related info in https://manual.macromates.com/en/scope_selectors.html