PSL is used to determine the level that a unique domain is registered at. This restricts cookies and privileges to that domain. It’s just a simple list because both .com and .co.uk are valid suffixes. Ios14 is using this list to prevent apps from tracking you across sites by limiting the data that can be stored per site. If you can get your domain recognized as a suffix as mysite.com then you can split information between all higher level domains. client1.mysite.com and client2.mysite.com. This allows you to store as much information as you want.