What is differential tracking system?

In differential tracking system set point changes with change in process value in one direction. E.g. there is trip of OMS in MEG plant on high oxygen flow (with diff tracker). Trip value is say 10% more than normal process value. Now suppose process value is reducing than trip set value will also reduce keeping 10% difference. If process value increased than trip set value will not increase and system will trip when trip set value is reached by process value.