One of the best-kept secrets concerning the ongoing detentions of non-citizens at Guantánamo is the Periodic Review Board (PRB) process being conducted pursuant to Executive Order 13,567. The PRBs, in a nutshell, are designed to provide (you guessed it) periodic internal review of the government’s continued justification for holding individuals who fall into either the “continued law-of-war detention” category (which has also been called the “too dangerous to release” category) or the “referred for prosecution” category. These two categories, in essence, are for all detainees who aren’t either (1) already cleared for transfer; or (2) already in, or convicted by, military commissions. In other words, the PRBs are designed to determine which detainees who are legally subject to continued military detention under the AUMF are nevertheless eligible to be transferred out of Guantánamo per the Obama Administration’s own criteria–because their detention is no longer “necessary to protect against a significant threat to the security of the United States”–and to regularly reassess each detainee’s case in light of changing circumstances.

The PRB process has not been without its warts. Indeed, there is currently litigation pending in the D.C. district court over the government’s slowness in even providing PRBs to some detainees. But for those detainees who have received PRBs, the (public) numbers are rather staggering. With the just-publicized news that a PRB has cleared Saudi detainee Mohammad al Rahman al Shumrani, that means that, of the 15 detainees whose cases we know have been heard by the PRBs, 13 have now been cleared. And that figure includes three detainees who, like al Shumrani, were initially denied clearance, only to have the PRB reach the opposite conclusion after a subsequent six-month file review as mandated by the Executive Order.

As a result, 13 different detainees have been moved from the most intractable “too dangerous to release” category (which has shrunk from 48 detainees in 2009 to 29 detainees today) into the category of detainees who can be sent home (or to third-party countries) once appropriate notifications are made and diplomatic steps are taken. The size of this latter group, if you’re scoring at home, now stands at 53. (The other 32 detainees include 22 “referred for prosecution,” seven currently in military commission proceedings, and three more serving sentences arising out of military commission convictions.)

To be sure, this figure could reflect selection bias–with the PRBs taking the relatively “weaker” cases first, and saving the detainees against whom the government has more evidence for later. But even if that’s true, the fact that 13 of the 15 detainees who were previously not approved for transfer have now had the government’s own process determine that their detention is no longer “necessary to protect against a significant threat to the security of the United States” speaks volumes about both why they were not approved for transfer previously, and what that might say about future PRB cases to come.

Of course, moving detainees into the “eligible for transfer” category is by no means a guarantee of their release. But it’s an important step toward that goal–especially for non-Yemeni detainees, of whom only a handful who have been cleared for transfer remain at Guantánamo. Moreover, just as we should not lose track of the myriad internal and external obstacles that continue to stand in the way of President Obama’s pledge to close Guantánamo, we should also not ignore one of the few truly salutary developments on that front–and the extent to which it may be a harbinger of more PRB clearances (and, thus, more detainee transfers) to come.