Bluetooth scans can either be passive or active. When passively scanning, a device will only listen to Bluetooth devices, quietly collecting data about its surroundings. Active scanners respond to every device they hear from, asking if they have more data to send. The devices then respond with any "scan response" data they have been configured to send.

Which Should I Use?

Generally, passive scans give you all the information you need and use less power than active scans, so are the best option. Active scans are only necessary if the Bluetooth device you are scanning for has been configured to send "scan response" data, you require said data, and the necessary integrations to collect that data have been implemented on the scanner.