The number shown on your app indicates the number of times your phone exchanged Bluetooth signals with other phones that have TraceTogether activated. The counter resets everyday.
Here's what happens:
1. The app logs all exchanges with nearby TraceTogether phones. Because Bluetooth's range can go up to 10 metres, phones close to you such as your neighbours' could exchange signals with your phone, through walls/ceiling/floor.
2. When a patient confirmed with COVID-19 uploads their Bluetooth proximity data to MOH, it is processed and filtered based on duration and signal strength to identify only the close contacts.
Filtering is not done on the phone because it's not a straightforward cut-off. The algorithm calibrates the data to account for different transmission powers of different phone models.