If your child fails one of their modules, they will have to retake the assessment (known as being referred). If they take a referral, the mark for that element is capped at 40% whatever they achieve. Their overall mark is worked out the same way as usual, by averaging the grades from all their assessments including both the ones they passed first time and the ones they've referred.
If they fail the assessment a second time they will have to take the whole module again from scratch. This includes attending all lectures and completing all the assessments whether they passed them previously or not. No marks will be brought forward from the last time they studied for the module and the overall module mark they can achieve is capped at 40%.
The Board may decide to let your child continue on to the next stage of the programme, as well as retaking the module they failed, if they only have to repeat 10 or 20 credits and they submitted all the assessments in the referrals for the module even if they didn't pass them. This is known as 'trailing' a module.