As others have said, HV contains ONLY sweet spot workouts due to the high volume. LV/MV utilize higher intensity workouts in order to push adaptations which would normally come from doing higher volume. I’m not sure why TR does this; it’s called SWEET SPOT Base – why am I doing VO2max work?!? Just admit you won’t get as much bang for your buck doing L/MV plans. Alas, the pitfalls of generic mass market products.
Not only that, but O/U are done for a very specific training effect – to develop your lactate tolerance/clearing abilities. Sweet Spot is all about building fatigue resistance and muscular endurance.
That said, the great thing about TR’s plans and calendar is that you can do whatever you want to suit your specific needs and requirements. I’m currently (and “technically”) in SSBHV2 but I’ve added a SS O/U workout session (e.g. Deerhorn) because it’s a perceived weakness I need to work on.