The 170 is a certificate of course completion and competence, it is not an exam. It's purpose is to inform the examiner who does your CPL or IR skill test that you have completed an approved course, met all the requirements for test and have a good chance of passing it - so he or she doesn't have to trawl through your training records to check for themselves. The person who signs the 170 form is not acting as an examiner and might not even be one. He or she is a senior instructor who has been authorised by the CAA to check that you are ready for test - so you don't waste your test fee and the examiner's time if you're not. It's all in CAA Standards Document 6 which is freely available on the website.