If you use this command, the creation wizard for a leaderboard will start.
You can create leaderboards that are the top 10 registered players in a category of your choice. Categories are: Total Mastery points, mastery points on a specific champion, all queue rank (best), Specific queue rank (solo/duoQ or FlexQ or TFT), Average KDA or Average KDA on a specific champion.
For leaderboards with specific champions you have to select the champion, search the list and send the numbers or just his name (e.g.: "28" or "Ezreal").