Community notes is the moderation tool that Elon Musk has promoted as a way for Twitter users to police the platform now called X for misinformation, but it has disappointed experts and irritated politicians over corrections judged abusive.

Announced early in 2021 -- two years before Musk bought the platform -- under the name Birdwatch, the feature initially allowed volunteers in the United States to signal questionable posts and provide notes with context.

These notes were initially just visible on a separate site, but then under the message themselves.

Twitter had just booted Donald Trump off the platform for inciting violent riots at the US Capitol and was seeking to promote healthier conversations.

In November 2022, a community note prompted the White House to retract a tweet that had exaggerated the impact of President Joe Biden's policies on retirement benefits.

Having become the platform's new owner, Musk relaxed moderation but still said his "goal is to make Twitter the most accurate source of information on Earth, without regard to political affiliation".

The publication of community notes has since been rolled out to 44 countries.

Twitter users who have registered as contributors write propositions for notes and they are not edited by staffers at X. They must nevertheless respect X's moderation rules.

The proposed notes are then submitted to other contributors. Those who get enough votes as being helpful may then be selected by an algorithm to be posted publicly.

The algorithm is inspired by the one Netflix uses to recommend content, and aims to "identify notes with broad appeal across viewpoints" rather than just ones that get the most votes as being helpful.

It is regularly updated, with one announced recently limiting the replacement of notes after publication.

According to X people are on average 30% less susceptible to agree to the contents of a post after having read accompanying community posts, and they are also less likely to repost it to their followers.

But Alex Mahadevan, the programme director for the Poytner Institute's MediaWise initiative promoting digital literacy, noted that the flaw in the algorithm is that for posts to be published they need to obtain a consensus across ideological divides.

 "Maybe that would have worked four years ago," he said at a June conference in South Korea in June.

"That does not work anymore because 100 people on the left and 100 people on the right are not going to agree that vaccines are effective."

What was required, he explained, was a "cross-ideological agreement on truth," and in an increasingly partisan environment, he said achieving that consensus is almost impossible.

Musk is considered by some to be in part responsible for that environment as, since taking over X, he has made deep cuts in content moderation teams and pulled the platform out of the EU's voluntary code of best practices on disinformation.

For Musk, community notes is an economical method to moderate content, including for political advertising which has recently been re-authorised on the platform.

For Mahadevan, the system has proven to be efficient in identifying and adding context to non-political content, such as pointing out AI-generated images, misleading advertising and already debunked conspiracy theories.

While the algorithm's complex mathematical formula is meant to discourage manipulation, it isn't foolproof and needs a large number of evaluators.

Julien Pain, the host of the "True or Fake" programme on Franceinfo, said there are groups in France that game the algorithm to promote their ideas.

He said hard right groups flood the platform with notes on posts by leftist politicians and try to get notes on posts by conservatives removed.

"Their contributions aren't always accepted but some get by," he told AFP.

"A note is interesting when it adds a bit of factual context to nuance a statement. But these just declare it's false and try to discredit the person," he added.

For French Green lawmaker Julien, the notes have become "a tool for ideological jousting which, unfortunately, can be distorted".