Under normal circumstances, the tier of entry restricts the grades which a student can achieve. On the foundation tier, the maximum grade is a 5 (or 5-5 for combined science), and on the higher tier, the minimum grade possible is a 3 or 4-3 (the minimum targeted grade is a 4 or 4-4, but a 3 or 4-3 is allowed for students who just miss a 4 or 4-3).
In cases where a centre’s centre assessment grades were adjusted upwards by the statistical model, then a foundation tier student with a centre assessment grade of 5 could have their grade adjusted to a grade not normally allowed on the foundation tier (i.e. grade 6 or higher). Ofqual decided that this higher, ‘off-tier grade’ should be allowed to stand, because changing it down to a 5 would affect the centre’s overall rank order.
Since grades are now protected so that students cannot receive results lower than their centre assessment grades, students on the higher tier would not have their grades adjusted down to a U from a 3 (or 4-3), because this would affect the centre’s overall rank order.
A full explanation can be found in the section starting on p116 of Ofqual’s report “Awarding GCSE, AS, A level, advanced extension awards and extended project qualifications in summer 2020: interim report”.