Intelligent Scan isn’t present in any stable Samsung software, nor is it found in any Oreo beta builds. Samsung describes it as a combination of “Iris Scanner and Face Recognition together for better results even in low or very bright light,” and even includes a video guide that has been helpfully uploaded to YouTube for your viewing pleasure.
Given the description and video, it sounds like Intelligent Scan will combine the sometimes-limited information the iris scanner and your front camera can each extract to piece together a fuller picture of whoever is trying to unlock the phone. The iris scanner might take priority in darker situations, but the facial recognition is still superior if your eyes aren’t aimed directly at the iris scanner or if they’re semi-closed. It’s also possible that it might simply be choosing one method over the other given your environment. The feature will likely debut on the Galaxy S9, though there doesn’t seem to be anything preventing it from being retrofitted to the Galaxy S8 and Note8 since it doesn’t appear to require any additional hardware. Assuming there won’t be any more leaks, there’s less than a month to go before Samsung tells us what Intelligent Scan officially does.