An X will show on the Attendance Certificate for any marks that will map to an X. This includes any Covid-related codes that begin X0-, such as X06. You can see a full list of these marks here: Attendance codes and how they count towards statutory attendance
Most places in Arbor (such as in your registers, on the Student Profile or in other reports) will show these codes specifically, rather than the X code.
As all of these codes map to X, they will display as X in statutory reporting such as in attendance certificates.
Any Illness marks, including I01, I02 and I, will map to an 'I' in the Attendance Certificate, not an X.