The Universities make S6 conditions contingent on the application's programme. There is no requirement on what AHs to take, but often a requirement to do Higher Biology in S6 if 3 sciences weren't taken at S5
Medicine , Dentistry and Vet Med DO have requirements about which AH to take, usuallly AH Chemistry and AH Biology. It would be quite hard to take AH in Biology in S6 if you hadn’t taken H Biology in S5.
If you hadn’t taken H Biology in S5, you would need to find a course that will take you with AH Chemistry , another AH and H Biology at S6. You would need to look at individual courses in Scotland to find out if there is one. It would work for Dentistry but not for Vet Med and I’m not sure about Medicine.
And none of these three courses require H Physics, unless it’s a substitute for H Maths. So there is no need for three sciences at S5 or S6. Which is just as well as many schools don’t offer that at S3/4.
And it doesn’t matter if 4 extra Highers would get more UCAS points than 2 or 3 AH. It makes much more sense for most pupils to do AH or a vocational course in subjects which are either a condition of their entry or will be useful on their course.
So if you have an unconditional offer for M Eng based on your AAAAB at S5, you could do AH Maths and maybe AH physics or Mechanics or a relevant course at college.
You wouldn’t “add breadth” and increase your UCAS points by doing H French, PE, Geography and Music .
If you have an conditional offer for Medicine, you will do AH Chemisty , AH Biology and AH / H in something else academic in which you are pretty sure of getting at least a B.
Which takes us back to the point of this thread, which I think was the implications of having fewer subjects at S3-4, which restricts your choices at Higher. This has consequences for courses ( like the ones I have mentioned ) which require the grades at Highers to be achieved at one sitting.
To get into these courses, you need to meet their grades by doing the required subjects AND then make sure your other choices are dead cert A. So having 7-8 to choose from is arguably better than 5-6.
However it may not matter for many other post school choices, as more Highers can be done in S6.