When you power on your SIM-based device, it performs a location update procedure. It will declare its International Mobile Subscriber Identity (IMSI) to the network.
The first location update is called the ‘IMSI attach procedure’. When the device is powered off, it performs an ‘IMSI detach procedure’ to tell the network that it is no longer connected.
When you switch the device on in a new area (for example, moving to a new country) or you change to a different operator's mobile network, the SIM must register with the new network to indicate its current location.