On the information that you are required to provide

Why am I asked for my location at the start or why do I need to state my address?
My provider is not listed in the selection menu. Does that mean that I cannot take part in the broadband test?
I am not sure what data transfer rate or what tariff type I have agreed. What shall I do?

Before and during the measurement

When I start the measurement, I get an error message saying JavaScript or WebSockets are not supported.
During the measurement I get a message saying an error has occurred.
Is there anything I need to bear in mind during the measurement? How can I create an optimum test environment?
What data volume is transferred during the measurement?
What part of Internet access is measured?
What factors can affect the result on App measurements?
What factors can affect the result on Web-Browser measurements?
At the start or during the measurement, I receive a message to the effect that the test request cannot be processed. What does that mean?
Despite heeding all advice, no successful measurement is possible.

On the measurement values

What data are stored?
What unit is used to indicate the connection speed? How is the conversion between Mbit/s and kbit/s carried out?
The test has been successfully performed and the test result is displayed. How can I save it?
The measurement result produced in the broadband test differs considerably from the contractually warranted data transfer rate. Is my broadband connection faulty?
How meaningful, in fact, are online tests?