Twitter is finally doing something popular: banning political ads. Yesterday, CEO Jack Dorsey announced that his company will no longer allow sponsored tweets promoting political campaigns. It will also prohibit the much broader category of “issues” ads, with a few exceptions. “This isn’t about free expression. This is about paying for reach,” said Dorsey. “Paying to increase the reach of political speech has significant ramifications that today’s democratic infrastructure may not be prepared to handle.”

The decision was widely supported, according to a survey from consumer polling group CivicScience. But it also sparked immediate debate, confusion, and uncertainty — because Twitter is trying to draw hard policy lines around the nebulous and sometimes all-encompassing realm of politics.

The current policy covers health care, climate change, immigration, and taxes

Twitter won’t lay out its new policy until November 15th. But the company already regulates political ads, so it won’t be starting from scratch. In mid-2018, Twitter laid out policies for both direct political campaigning and “issue ads” in the United States. Account holders need to apply for certification, which lets Twitter verify their identities, and their promoted tweets will be specially marked — either with the name of a campaign or with the general label “issue.”

Issue advocacy includes two categories: “ads that refer to an election or a clearly identified candidate” and “ads that advocate for legislative issues of national importance.” Twitter identifies “abortion, healthcare, guns, climate change, immigration, [and] taxes” as examples of legislative issues, although that’s not an exhaustive list.

Twitter maintains a list of certified “issue advertiser” accounts and their campaigns — which offers a little more insight into what the new policy might cover. It includes well-known advocacy groups like Planned Parenthood Action and FreedomWorks, as well as the lobbying branches of big companies like Verizon. You’ll also find government accounts pushing tweets about civic participation — like the New Jersey Department of State urging people to complete the Census.

Twitter policy head Vijaya Gadde reiterated the current definition when OneZero reporter Will Oremus asked what would constitute an “issue.” But she noted that “we are working through the details now and we will provide more details on the final definition” in November. Dorsey already confirmed that Twitter will allow “get out the vote” efforts — an exception Facebook previously established in France, where most political ads are already banned. Gadde explained that the final rules might have other exceptions, but they’ll be “well-defined” and rare.

There’s already a fight brewing over one big potential carveout: news outlets. When Twitter introduced its certification rules last year, it exempted news organizations that met certain standards, reasoning that they “report on these issues, rather than advocate for or against them.” Qualifying outlets must have 200,000 monthly visitors and offer information about their editorial staff, they can’t be primarily composed of user-generated content, and they can’t be “dedicated to advocating on a single issue.”

President Trump’s reelection campaign manager, Brad Parscale, has questioned whether this policy will continue. Parscale claimed Twitter’s decision was meant to censor conservatives, suggesting that the company would still allow “biased liberal media outlets” to “run unchecked.”

Trump’s campaign manager wants news organization ads banned, too

Twitter hasn’t confirmed whether it will ban “issues”-focused ads from media outlets; a spokesperson told The Verge that “we’ll know more details” on November 15th. News organizations have a huge presence on Twitter, so banning these ads would be a huge deal. It would effectively bar many sites from advertising their biggest stories, especially as the 2020 election approaches. At the same time, Twitter’s current language, which focuses on impartiality, leaves it vulnerable to attacks — and it highlights one of the core problems with Twitter’s decision.

It’s almost impossible to discuss many political topics without implicitly taking a side. If news organizations describe climate change as an emergency, for example, they’re siding against American politicians who deny it exists. Defining an “issue” has proven messy outside the news media as well. A bookstore with a Facebook page reportedly had trouble advertising an event featuring author Ijeoma Oluo and her book So You Want to Talk About Race, since it was tagged as political. Facebook and Google banned political ads entirely in Washington, thanks to the state’s complex campaign finance laws. The result was an inconsistently enforced policy that infuriated several candidates.

And Twitter — like every large social media platform — has struggled with all kinds of moderation. It’s promised to moderate political figures that break its rules, but rarely enforced that policy. A well-meaning ban on hate symbols led to Twitter suspending an academic for promoting his book about hate groups. Even the best-written policies are difficult to apply across a site with hundreds of millions of users.

We don’t know how much Twitter’s ad ban will matter. Organizations can still use shady tactics like buying followers to amplify their reach. Misinformation can easily spread through viral non-promoted tweets. And Twitter’s main goal may be simply to make its rival Facebook look bad. Dorsey announced the news just before Facebook’s quarterly earnings call, following a week of furious debate over whether Facebook would ban or fact-check political ads. (The answer is “no” on both counts.) Even if it’s symbolic, though, Twitter’s policy could still set the tone for what “reasonable” political content moderation looks like — or prove that defining politics is a lot harder than it looks.