Final answer:
A grease pencil is used to mark doors or create messages on the back of a clipboard that can be flashed to crews.
Step-by-step explanation:
The item used to mark doors or create messages on the back of a clipboard that can be flashed to crews is a grease pencil. It is a type of wax pencil that is commonly used in various industries, including firefighting, construction, and manufacturing. Its waxy texture allows it to write on different surfaces, making it ideal for marking doors or leaving temporary messages.