text: System R also derived from Codd 's theories , and Ellison wanted to make Oracle 's product compatible with System R , but IBM stopped this by keeping the error codes for their DBMS secret .
simplified: System R was also based on Codd 's theories . Ellison wanted to make Oracle 's product compatible with System R , but IBM stopped this by keeping the error codes for their DBMS ( DataBase Management System ) secret .