The Articles of Association is the founding document that lays out:

• What CSU is 
• What CSU does 
• How CSU is governed, by how many people and in what capacity
• Who qualifies as a member
• How to call referenda (student-body-wide votes on policies)
• How General Meetings are called, and how they qualify as General Meetings
• The powers and responsibilities of the Student Council, and,
• How minutes and records should be taken, stored and communicated

It's a long but important document. You can read it in full here.