Aggregation is when the marks from different units are added up to calculate what overall grade you will get. It can also be called certification or cashing-in. For A levels, the cash-in request is normally made at the same time as the entries (when it's free) though it can be made up to about a month after the results come out. For GCSEs it has to be made at the time of entry. It's not really something that happens 'during exams' unless a mistake has been made earlier.