One of the information provided by the webletter (or email in the case of DDTs), prepared by the Observing Programmes Office (OPO), together with the final comments to your proposal is also the priority ranking.  Indeed, all programmes allocated SM observing time are assigned to one of the following priority groups:

• Group A: High Priority: These programmes are considered to have the highest scientific value and are executed first as observing conditions allow. ESO makes every possible effort to complete programmes in this Group. This includes the possibility of carrying over programmes in this category to the next semester, in case that their degree of completion at the end of the allocated semester is too low for the fulfillment of the scientific goals. 

• Group B: Medium Priority: Programmes in this group have lower scientific priority than Group A and are executed only when no Group A programme can be executed. ESO tries to complete all programmes in this Group, but incomplete programmes are terminated at the end of the allocated semester. 

• Group C: Low Priority: These programmes have lower scientific priority than those in Groups A and B but have relaxed constraints, which allows them to be scheduled when the external conditions are not suitable for the execution of any programmes in Groups A and B.