If you have created more than one player, the reEmbed script will need to know which player to use. Player Rules allow you to define URL paths for each player to appear on.
For example, your site has a category "Sports" and a category "Tech", and you've created a separate player for each category. Assuming your site uses the following URL structure:
You can add the following rules to assign the correct player to each category
Sports player => /sports/
Tech player => /tech/
The reEmbed script will try to match a rule to the URL of the current page, and if it matches a rule, it will use the corresponding player.
In the case that no rules are matched, your default player will be used.