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.  
On the form you'll see a couple of abbreviations:
IB stands for Item Before that check in the Fed file that we receive
IA stands for Item After that check in the Fed file that we receive
SOR resolves most research issues and is needed to track the progress of a check from the beginning (depositing bank) to its final destination (the paying bank). It is very common for banks or FRB offices to request SORs for checks they are researching.
To obtain a SOR, fax a check copy request to the Item Processing Dept (616-285-5705) and write "Please Provide SOR" on the request.