WAF rule: web server list

Hello All,

I have a question regarding WAF rule. Whhile creating WAF rule we got option to select web server list. We can add more than one web server.
If we have more 3 web server how it works. I mean does request goes in round robin way (first request to first web server and second to second).
There is an option called change priority as well. Which makes me think how the request are handled by the rule.

https://community.sophos.com/kb/en-us/126470

Thanks.