MI20 (Print List of Differences) is a standard SAP transaction code available within R/3 SAP systems depending on your version and release level.
Below for your convenience is a few details about this tcode including any standard documentation available. In-order to use this transaction within your SAP system simply enter it into the command input box located in the top left hand corner and press enter. Here are a few additional command options available to use when doing this.
SAP Menu->Logistics->Materials Management->Physical Inventory->Difference->Difference List
SAP Menu->Logistics->Logistics Execution->Internal Whse Processes->Physical Inventory->In Inventory Management->Difference->Difference List
SAP Menu->Information Systems->General Report Selection->Materials Management->Physical Inventory->Print List of Inventory Differences
For futher details and documentation see program RM07IDIF.