Formal and Official:
* Order: A command or instruction.
* Directive: A formal instruction.
* Command: A formal order.
* Edict: A formal proclamation.
* Decree: A formal order or decision.
* Instruction: A formal direction or order.
Informal and General:
* Requirement: Something that is needed or demanded.
* Demand: A strong request.
* Requirement: A thing that is needed or essential.
* Call: A strong appeal.
* Duty: A moral obligation.
* Responsibility: A task or obligation.
The best synonym will depend on the specific context. For example, "order" might be more appropriate for a formal legal document, while "requirement" might be better for a casual conversation.