What is the use of set control field in record field properties?

Set Control id is used when you want to share tables in Peopletool applications.
This key identifies the sets of information in the table that are shared by multiple companies or business units under your corporate umbrella. You then specify a set control field, which identifies which fields map between the original key and the TableSets. You can specify any field that logically identifies the TableSet. In this case, you might assign Company as the set control.