AS and A2 can be added together to make a whole A level (unless you only do AS and not A2 for a subject)
If an A level has a total of 600 points. If you get say 200 for AS and 300 for A2 you will have 500 points. So sometimes people redo the AS exam to secure more points. If you redo it and do worse the highest scoring time counts.
I don't know what amount of marks are required for an A. Universities will look at how many points in total you have to see if you are worth interviewing.
You can also get points from extra-curricular activities and such but no one does them and GT4 doesn't count so ignore that.

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I think thats all correct. meh