This is the 2167th most frequent Portuguese word.
obrigatória
Mandatory; obligatory.
Here, 'obrigatória' is used as an adjective to indicate that reading the manual is a condition that must be met by the participants.
A leitura do manual é obrigatória para todos os participantes.
Reading the manual is mandatory for all participants.
In this case, 'obrigatória' again functions as an adjective, specifying that attendance in the class is required, showing obligation imposed by someone.
A presença na aula de hoje foi considerada obrigatória pelo professor.
Attendance in today's class was considered mandatory by the teacher.
This represents a legal or regulatory requirement, with 'obrigatório' inflected to agree with the masculine noun 'uso'.