Registration for assessment periods (regular and appeal) is automatic. By registering for the curricular unit for the academic year, the student is registered for the normal period. If you fail the course, the system will automatically register you for the appeal period. However, it is still necessary to register for grade improvements and for the PEE, which must be done by the students.
The use of the exam in the normal period presupposes compliance with the RGAC (Regulamento Geral de Avaliação de Conhecimentos) - Article 2 - Point 7. As such, you will only be able to take the exam in the normal period when the assessment criteria for the course unit allow it. That is, when the weighting given to the assessment throughout the semester allows you to take the written test in the normal period (you should consult the assessment criteria for each course unit on its page).