Facebook first announced the features back in December, and have slowly rolled them out since then. Users who have them enabled can be notified of photos they appear in, even if they haven’t been tagged, so long as the photos’ privacy settings allow it.

When we reached out to Facebook for more information on how the face recognition features work, they referred us to two blog posts from December announcing the new settings and addressing privacy concerns. Reading those statements closely can tell us a lot, but there are some things we still don’t know. Like, just what, exactly, Facebook wants to do with our faces.

How does it work?

When a Facebook user with either “tag suggestions” or “face recognition” turned on is tagged in a photo, the social network’s machine learning systems analyze the pixels of the face in the image, creating what’s called a “template.” Facebook describes the template as a “string of numbers,” but each user is assigned a template that’s unique to them.

When new photos are uploaded, Facebook compares faces present in the image to templates of relevant users and suggests a tag if there’s a match. Because it’s both unique and able to identify users, it’s helpful to think of the template as a sort of thumbprint.

Facebook was clear that these templates are voluntary: You only have a template if you have tag suggestions or face recognition enabled. If you decide to turn off tag suggestions or the new face recognition features, Facebook says the templates are deleted. However, you can re-enable the features.

What do I get out of all this?

If your template pops up in a newly uploaded photo, Facebook’s new face recognition features can send you a notification, even if you haven’t been tagged, so long as you have the features enabled and are part of the photo’s target audience. If you have an active account, the new features can also tell you if another user uploads an image of your face as their profile picture and flag that user for catfishing. Additionally, visually impaired users can use face recognition in conjunction with the social network’s automatic alt-text tool to identify who appears in Facebook photos. Undeniably, these face templates are a powerful way to identify people and detect fraud.

Okay, I’m in, how do I turn it on?

The new face recognition features haven’t rolled out to all users, and won’t be available everywhere, but you can check to see if they’re available to you by selecting “Settings” from the triangle-shaped drop-down menu at the top right of every Facebook page. If there’s a column called “Face Recognition,” simply click that and then agree to use the service.

Additionally, if you already have tag suggestions turned on, the new face recognition features will initially be enabled as soon as they roll out to you.

Sounds weird, how do I opt out?

If you have tag suggestions turned off, Facebook says that the new facial recognition features will be turned off by default when they’re made available to you. Your tag suggestions status can be found by clicking “Timeline and Tagging” on Facebook’s settings page and looking for “Who sees tag suggestions when photos that look like you are uploaded?” If it’s set to “Friends,” it’s enabled. If it’s set to “No one,” it’s off.

Users who have tag suggestions or face recognition enabled can also opt out of them at any time (using the processes described above) and their templates will be deleted, according to Facebook.

So what does “opt out” really mean?

Facebook says that it “respects people’s existing choices” on face recognition. If you had tag suggestions off, face recognition is turned off. If you had tag suggestions on or set to friend, face recognition is on.

However, Facebook confirmed that when tag suggestions were first rolled out, the template system was enabled by default—everyone was enrolled until they opted out. Thus, as Facebook rolls out the new features, many users will be automatically enrolled by virtue of simply never having changing their settings. If you have an older profile and you’ve never once changed your privacy settings, you’re enrolled in the new face recognition features until you opt out.

Turning off the setting deletes your template, thus removing you from both the tag suggestion and new face recognition processes entirely, according to the company. Facebook states that the new features aren’t available in Canada or Europe “where we don’t currently offer face recognition technology.”

What does Facebook see?

The specifics of opting out are important because, theoretically, Facebook has the power to recognize hundreds of millions of people in photos—even if we have no reason to expect them to do so. As my colleague Kashmir Hill wondered, what if you’re simply caught in the background of a photo by happenstance, would Facebook ID you? Or make some inference about you based on who you were with? Facebook denies it’s using face recognition this way, but we need to be cautious about their ability to biometrically identify and catalogue scores of people in an instant.

What doesn’t Facebook see?

Biometric data is extremely precious and more people are moving to protect it. As we mentioned earlier, Facebook is currently being sued for allegedly violating Illinois’ Biometric Information Privacy Act. The plaintiffs claim that Facebook’s widespread collection of face data violates BIPA. Facebook fired back, saying that while they do scan for face templates, this collection doesn’t actually harm anyone. Facebook tried to have the case dismissed on those grounds, but it’s still moving forward. Facebook could pay up to $5,000 for each violation.

Since Illinois passed BIPA in 2008, several other states have followed their lead, with similar laws now on the books in Texas and Washington. Those states, and countries with their own digital privacy laws, may be the only places in the world where users truly own their own faces.

Meanwhile, collection of face data by a wide array of technology companies rolls ever onward, with our faces being used for everything from identification at airports to customer reward points for free burgers. Perhaps the best summary of the high stakes involved comes from Adam Harvey, a counter-surveillance expert who spoke with Gizmodo last fall.

“When any information is co-opted for security purposes it becomes less secure to share,” Harvey said. “For example, sharing your mother’s maiden name online would not be a good idea. Likewise, Facebook’s proposed facial recognition product would make sharing your face online a security issue, even more so than it already is.”

Correction: An earlier version of this story incorrectly described the new face recognition features as solely opt-in. In reality, the features are enabled by default if a user previously had tag suggestions enabled. The story also incorrectly stated that Facebook can “automatically tag” users. In reality, it can automatically suggest tags. We regret the errors.