The PSU is governed by its Constitution. You may download it here.

Article 4. Governance of the PSU
4.1 The Governance of the PSU shall be based on the democratic principle that every member shall have the fullest opportunity to directly participate in PSU affairs.

4.2 Subject to this constitution, the powers of government of the PSU shall be vested in, and exercised by the organs established by this constitution; namely in descending order of authority:
1° Referendum
2° General Meeting
3° PSU Council
4° PSU Executive

4.3 The PSU is committed to transparency at all levels of the decision making process, and will be releasing minutes and informing the postgraduate community of all decisions taken at PSU meetings.

The PSU Constitution in full can be accessed here.