How the aggregate works
Each grade carries a number, from A1 as one upward. Because the numbers rise as performance falls, a lower aggregate is the stronger result — the opposite of most scoring systems, and a common source of confusion.
Which subjects count
The three core subjects always count, however they went; there is no discarding a weak core grade in favour of a strong elective. From the electives, the three best count. If you sat four, the weakest is set aside — which is where most people miscalculate, usually by including a subject they feel attached to rather than the one that scored better.
The aggregate is not the whole story
Programmes set subject requirements as well as cut-off points. A qualifying aggregate with the wrong combination will not secure a place on a programme that requires specific electives, and no amount of aggregate strength substitutes for a missing prerequisite.