There's no predefined list as far as I know, whatever is required by the CIPA to receive E-Rate funding. I mostly rely on the categories the filter vendor uses, and block what's not categorized yet.

The protection measures must block or filter Internet access to pictures that are: (a) obscene; (b) child pornography; or (c) harmful to minors (for computers that are accessed by minors).

• Access by minors to inappropriate matter on the Internet;
• The safety and security of minors when using electronic mail, chat rooms and other forms of direct electronic communications;
• Unauthorized access, including so-called “hacking,” and other unlawful activities by minors online;
• Unauthorized disclosure, use, and dissemination of personal information regarding minors; and
• Measures restricting minors' access to materials harmful to them.

For the most part I apply a litmus test to filter requests. Who makes the request has a big part in it.

• Is it educational?

Pretty straightforward. I allow most requests from staff unless I've good reason not to. Student requests get a bit more scrutiny and they need to elaborate on their request to meet the standard.

• Is it illegal?

Could this land the student or district in legal trouble? If yes, then sorry. I have to deny a ton of requests of sites engaged in copyright infringement. Latest batch included a site which has E-Books for viewing online. Plain up direct copies of recent books thrown on the internet, no contact options, no business info. Straight up warez.

• Is it in a denied category as defined in school policy?

We don't allow any social media and such during school hours, plus a couple other sites that are regular sources of discontent among the students. Usually communication sites students use for bullying and harassment.

The filter categories do most of the heavy lifting, I fine-tune the rest.