The drop in air temperature across an evaporator coil (return minus supply).
Temperature split is return air temperature minus supply air temperature across the coil. It gives a quick read on how much heat the coil is removing, but it depends on airflow and indoor humidity, so it is a check, not a charging method. A split far from what the manufacturer's data predicts points you toward airflow or refrigerant problems to investigate with proper measurements.
Exam work areas
E · Commissioning and start-up F · Maintenance, service and troubleshooting