The JCR constitution describes the functions and the running of the JCR. It dictates how money is spent, how an election is run, the duties of the committee, the rights of members, and much more.
Changes to the constitution can only be proposed at two points in each year, in special constitutional change meetings. One of these meetings takes place in Michaelmas term, and the other in Trinity. More details on changing the constitution can be found in article VIII.
The JCR can reimburse members for costs involved in committee duties, JCR events, and for society funding and purchases that have been ratified by a JCR motion.