Yearly report where you sum up what you have done so far, the conferences you went to, the other tasks you were responsible for (teaching, administration, etc), your publications, your plans for next year and your feedback for and from supervisor/advisory committee. This will be evaluated by the (interdisciplinary or facultary) PhD Progress Monitoring Commission.
Also if you are doing a joint PhD and the other university is the main institution, you need to hand in a progress report at VUB!
Check the regulations art 23-24
Check the faculty form and deadline (expect an email from the faculty about the practical details)