What is the "plain error" standard for appellate review of a defendant's application for a reduced sentence?

Under these circumstances, we see no reasons to deviate from our general rule that the plain-error standard governs the review of unpreserved errors. See Vonn, 535 U.S. at 73 (noting that failure to review unpreserved errors under the plain-error standard creates a perverse incentive for defendants to "simply relax and wait to see if the sentence later struck him [or her] as satisfactory"); see also United States v. Sanya, 774 F.3d 812, 815 (4th Cir. 2014) ("Because [defendant] neither objected to the judge's involvement in plea discussions, nor made an attempt to withdraw his guilty plea, we consider his appellate argument under the rigorous plain error standard.").