A screed rail is used in conjunction with the screeding level and other tools used to lay an even floor.
Screed rails are used to guide the levelling tool. They also section areas to be covered and help to set the depth for the new floor.
Many things can be used as a screed rail: wood, metal pipes or bars, straight edges and even other screeding levels. These are usually removed after levelling.
There are manufactured screed rails which are not designed to be removed. These are often used in large industrial spaces. They are typically made from plastic and their design helps to prevent cracking.