Courses that have been already attended must be included in the Study plan before the courses still to attend. In the list of attended courses you will find  as well the ones that you have already attended and passed but not registered yet: you are required to include these courses newly in the Study plan. 
When courses included in the Study plan exceed the yearly limit of 80 CFU, the courses included lately are marked in red (overbooking). As soon as exams are passed and registered, the relative courses included in the Study plan are cancelled and the credits that are now “free” can be allocated to the courses marked in red respecting the order of inclusion. 
The overbooking process is applicable only to exams passed within the Autumn examination session. 
Please bear in mind that the courses not included in the Study plan can not be attended neither taken. 
It is responsibility of students to pay particular attention during the definition of the Study plan in order to include the courses willing to attend in order of priority.