What is the purpose of the rear buffer conveyor belt in the machine code KM-ASSYM5-07?

What is the function of the M91 error code in the machine code KM-ASSYM5-07?

What is meant by "operation timeout"
The solution for the M91-Rear buffer conveyor belt discharging operation timeout error may involve several troubleshooting steps. These steps could include checking the conveyor beltâ€™s condition; enabling or disabling the rear buffer convey