Questions relating to the defense of a thesis and the process surrounding it, such as the grading system of a thesis, who may come to a thesis defense, the duration of a thesis defense session, etc.

According to caltech

The main purpose of the [...] Thesis Defense is to have a committee of faculty examine the doctoral candidate on the scope of the thesis and its significance in relation to the major subject.

Some further detail from The University of New South Wales

The thesis defence or viva is like an oral examination in some ways. It is different in many ways, however. The chief difference is that the candidate usually knows more about the syllabus than do the examiners. Consequently, some questions will be sincere questions: the asker asks because s/he doesn't know and expects that the candidate will be able to rectify this.