A constitution of a federal system is usually written and rigid.
In a federal system, there are two levels of government: the central/federal government and the state/regional governments. The constitution outlines the powers and responsibilities of each level of government, as well as the relationship between them.
A written constitution means that the constitution is in a formal, written document that outlines the fundamental principles and laws of the federal system. A rigid constitution means that it can only be amended through a special process that is usually more difficult than the normal legislative process, which helps to maintain the balance of power between the different levels of government.
Therefore, the constitution of a federal system is usually written and rigid to ensure that the powers of the federal and state governments are clearly defined, and to provide a stable and predictable legal framework for the citizens and governments of the federation.