The best portfolio evidence depends on the subject area. Open Doors is not judging every applicant through one generic scholarship lens. It is judging whether your background fits the academic area you selected. That means the same certificate can be useful for one subject and almost irrelevant for another.
For Applied Mathematics and Artificial Intelligence, Computer and Data Science, Engineering and Technology or Physical Sciences, useful evidence can include coding projects, GitHub or project documentation, mathematics awards, lab work, research posters, technical internships, engineering prototypes, data-analysis work or strong grades in quantitative subjects.
For Business and Management, Economics and Econometrics, Politics and International Studies or Education and Psychology, useful evidence may include research papers, policy writing, case competitions, leadership in serious projects, internships, debate, community programs, teaching experience, data analysis, publications or strong coursework in the field.
For Biology, Biotechnology, Chemistry, Materials Science, Clinical Medicine, Public Health, Earth and Environmental Sciences or Urbanism, stronger evidence may include lab experience, field work, clinical exposure, health volunteering, environmental projects, research abstracts, conference certificates, technical reports or documented project results.
The goal is not to force every activity into the portfolio. The goal is to help the reviewer quickly understand that your subject choice is real. If the document does not support your academic direction, leave it out or place it lower in priority.