Stopping a sensor in xDrip+ has very different functions depending on the sensor itself.
G4 with bridge, G5 and G6⌁
G5 and G6 reference documentation is here.
xDrip+ will actually send a sensor stop command to the transmitter, the same way it would be done by the receiver or the vendor master app.
Never stop a G6 sensor unless you actually want to remove it.
If you want to restart your sensor, make sure the transmitter supports this option: recent G6 transmitters require to be physically removed for this operation.
Go to the system status G5/G6 page and wait until you see the sensor status is stopped.
You can now remove the transmitter and discard the sensor.
If you need to change transmitter, make sure you have connection before actually starting a new sensor.
xDrip+ is not able to stop a Libre sensor, actually you can't stop this sensor: if it doesn't fail it will run it's useful lifetime and stop by itself after 14 days and 12 hours.
xDrip+ sensor stop is only used to invalidate previous calibrations. This operation is necessary and recommended each time you replace your sensor.
STOP SENSOR unless you only want to reset all calibrations.
Confirm you want to stop the sensor.
If you change your mind and the sensor didn't expire yet, you can always "restart" it, il will resume the current session and eventually ask for a new calibration. xDrip+ doesn't physically restart Libre sensors.
Followers and companion apps⌁
There is no interest in stopping a sensor when using a follower app.
The only exception is xDrip+ Sync follower that might transmit the Start or Stop command to the actual sensor if it is directly connected to xDrip+ acting as master.
Do not use Start and Stop sensor with xDrip+ Sync follower unless you know the person with the master xDrip+ phone will know what to do with a stopped sensor!