You can find the information on how to progress from A license to Pro or World Champion in all 4 categories, on our Documents and Tools page.
https://members.iracing.com/membersite/member/instruction.jsp
On the UI, you can find this page by clicking the Help and Support tab, then click on More Documents.
On the members site, hover over help, then click on Documents and Tools.
There is a section dedicated to Championship Racing. Each series is labeled with it's respective category icon, ie: Road, Oval, Dirt Oval, Dirt Road
Please know that if you find series information that is out of date, the season has ended and it will be updated once the series details are finalized for the new season.
This information will also be published by a staff member in the forums.
You may also find additional information by visiting https://www.iracing.com/membership/ then hover over the eSports drop down. Click on your chosen series.
Each series follows it's own rules and specifications and are subject to change from season to season.