The Use of Sarum was basically the variety of the Roman Mass that was used in Salisbury, England prior to the Reformation. It differed a little--though not much--from the Mass as said in Rome at the time.
Textually, you are somewhat correct regarding the Sarum vs. Roman use; however, the rubrics were remarkably different.
Sarum had its own liturgical color schema and own liturgical calendar that differed in many respects from Rome, it also had a number of offices not found in the Roman curial usage, aka the Tridentine rite.