pen vs pencil in tournament

I believe I saw the contestants using pens in the early rounds of the tournaments although the 20-year old mentioned that he incorrectly filled the same spaces twice with the wrong answer, so I was wondering - if they use pen, do they just go write over their mistakes and hope the judges can read the overprinting?

In the finals, they switch to magic markers and I'm presuming on an erasable background. Seems kind of weird that the semis are non-eraseable and then the finals are eraseable.