The first line contains the file path (relative to workspace root for saved files, or just the filename for untitled files), followed by the selected code with line numbers.
Usage
Select the code you want to copy in the editor
Open the Command Palette (Cmd+Shift+P on macOS or Ctrl+Shift+P on Windows/Linux)
Type "Copy Selection with Line Numbers" and select the command
The selected code with line numbers is now copied to your clipboard