TL;DR
Initially, three categories of speed tests were distinguished (realistic, mainstream and conservative). Later, two minor categories (ambiguous and miscellaneous) were added.
Realistic speed tests
A realistic speed test gives an indication of the true internet speed provided by your ISP.
A speed test is considered as a realistic speed test when the measured internet speed is 109-112% of the advertised speed.
Mainstream speed tests
A mainstream speed test gives test results that are close to the advertised speed. That is the measured internet speed is 100-103% of the advertised speed.
Conservative speedtests
Conservative speedtests measure structural an internet speed lower than the advertised speed.
Ambiguous speedtests
Ambiguous speedtests are speed tests of which the results are 104-108% of the advertised speed.
Miscellaneous speedtests
The last category of speed tests are speed tests that doesn't match the criteria above.