When you go to book your exam on the RSGB website, you’ll see a box labelled “Candidate Number (if known)”. This is to help the RSGB find you on their system.
When you book an exam, RSGB issues you with a candidate number. So, the very first time you book an exam, you won’t be on the RSGB system and won’t have a candidate number yet – in that case, leave the box blank.
If you are taking a second exam (either a retake or an Intermediate/Full exam), then you’ll be on the system and have an RSGB candidate number.
As far as the form is concerned, if you have been given a number by RSGB, then put that number into the box. If you don’t have a number yet, or can’t find the number that RSGB has issued you before, just leave it blank.