I've read what seems like 30+ articles and explainers about the UK law the last few days - this has some lousy (official) defintions. I think the most recent episode of Power User with Taylor Lorenz might cover some of this enough to get the overall sense.

The topics under scrutiny of the "user-to-user" site is extremely vague beyond obvious porn, but it amounts to if it allows the sharing of links of basic news of any topic, it counts. Because in terms of categorizing "harmful content" for minors, seeing fucking protests happening anywhere, at all is "controversial adult content." But if the links are limited to a very specific topic, say Honda Ridgeline owners, privacy and cyber nerd shit no one cares about) etc., cooking, and other innocuous things, it's a grey zone that doesn't demand compliance. YMMV, but even for a fascist wannabe set of policies can't justify "harmful" material for kids with a Linux forum or a forum for owners of the Honda Ridgeline (WTF?)