Extended certification works in two ways. 
 

If you perform particularly well, you can get credit at a higher level on the CEFR.
 

For example, in Cambridge English: Advanced (formerly known as CAE) if you get grade ‘A’ in the exam, you receive a certificate indicating that you are at C2 (proficiency) level. For grades ‘B’ and ‘C’ candidates will receive a certificate at C1 level.
 

You can also receive credit for your English language skills, even if you do not achieve a passing grade. So if you do not get enough marks for a grade ‘C’ in the exam, you can still be awarded a certificate showing performance at level B2 if you show this level of ability in the exam.
 

(Grades ‘A’, ‘B’ and ‘C’ represent a PASS grade. Grades ‘D’ and ‘E’ are FAIL.)
 

Another example: in Cambridge English: First (formerly known as FCE) if you pass at grade ‘A’ in the exam, you receive a certificate that notes that you have achieved C1 (advanced) level. For grades ‘B’ and ‘C’ candidates will receive a certificate at B2 level. A candidate who fails with a ‘D’ grade may receive a certificate that notes they have achieved B1 level.