Home Assistant for iOS receives significant location updates from iOS. Whenever an update is received, it is sent to Home Assistant. Roughly, an update is received everytime that your device transfers to a new cellular tower, a significant amount of time has passed (usually a couple hours) or a connection state changes and the system notices your location recently changed.
Apple defines significant significant-change location updates as:
The significant-change location service delivers updates only when there has been a significant change in the device’s location, such as 500 meters or more.
They also say in the Energy Efficiency Guide:
Significant-change location updates wake the system and your app once every 15 minutes, at minimum, even if no location changes have occurred.
Finally, I think this answer from Stack Overflow says it best:
The significant location change is the least accurate of all the location monitoring types. It only gets its updates when there is a cell tower transition or change. This can mean a varying level of accuracy and updates based on where the user is. City area, more updates with more towers. Out of town, interstate, fewer towers and changes.
What’s the real story on significant-change location updates? Who knows, because Apple keeps it private.
At launch, Home Assistant for iOS sets up geofences for all zones in your Home Assistant configuration. Enter and exit notifications are sent to Home Assistant.