From the link Scary posted above.

Thank you Tinkers. I saw that but it didn't make much sense to me as there are no assessments anymore are there? I will head over to JCQ to find out more, and then probably ask more questions!

Where the student misses one of the exams (e.g. say paper 2 of 2) in these cases, "a mark is calculated for the missing unit provided the student has completed enough of the specification to meet the minimum requirements specified by JCQ (the 25% referred to in Tinkers post). The calculation of the missing mark takes into account the student's performance in the other comparable units of the exam and the national average for those units. This method is considered to be fair and consistent". So they would look at how well they did in the other exam(s) or that subject and give them the same percentage depending on whether the national average achieved for that paper was the same as the paper they sat. If you see what I mean.
This would suggest that if your DC has a doctor certifiable condition that flares up on the day they are supposed to sit the exam, they might be better not sitting it (providing they have done ok on the other exams for the subject) rather than sitting it, performing badly and getting a 5% max adjustment. But you would need very good medical support for this.

So they can't miss exams...

The adjustment for assessments is for where they miss an exam - they have to have done at least 25% of the assessment (ie 25% of the exams for that course, if it is a fully examined course) and have robust medical evidence for why they missed up to 75% of the exams for that course, to be entitled to a calculated result. This takes into account mock results plus a complicated analysis of their actual exam result for the paper/s they sat and the national averages.

This is why mock results are important. For example a friends son's appendix burst in one of his Maths exams - he missed 3 exams that week (3 different subjects) but because he had completed all the other exams for all those subjects his results for those 3 subjects were calculated.