It is explained on the websites of the exam boards, also on the JCQ site.
www.jcq.org.uk/Download/exams-office/post-results-services/post-results-services-june-2019-and-november-2019
Possibly the most pertinent may be:
Priority Service 2 (Review of marking)
The service is available for externally assessed components of both unitised and linear GCE A-level specifications.
This is a priority post-results review of the original marking to ensure that the agreed mark scheme has been applied correctly.
A marking error can occur as a result of:
• an administrative error;
• a failure to apply the mark scheme where a task has only a ‘right’ or ‘wrong’ answer;
• an unreasonable exercise of academic judgement.
The awarding body will train its reviewers to conduct reviews of marking accurately and consistently. Reviewers will not re-mark the script. They will only act to correct any errors identified in the original marking
I'm surprised that your school hasn't given you / the candidates the information already, though?
(Although, come to think of it, although we have had the relevant details from DS2's school re GCSEs, if there has been anything from DD's wrt A levels, either I've missed it, or she has failed to mention. All scenarios are possible, tbh. I think I resorted to looking at the exam timetable on DS2's school website to find out when DD's exams were
).