Source of Receipt (SOR) is the paperwork that accompanies the checks to the Check Processor. It is the documented proof of when and from whom the checks were received. SOR provides the ABA# of the bank the checks are received from, the date of the cashletter, the amount of the bundle the check was in, the position of the check in the bundle(checks before & after), and the tracer number assigned to the check.
SOR resolves most research issues and is needed to track the progress of a check from the beginning (depositing bank) to it's final destination (the paying bank). It is very common for banks or FRB offices to request SOR for checks they are researching.
To obtain SOR, fax a check copy request to the IP Dept (616-285-5705) and write "Please Provide SOR" on the request.