It is often discussed...
over a glass of beer what exactly is top or bottom fermentation. For some people, the term refers to the temperature that the beer ferments in the tanks. Although it is a coincidence that bottom fermented beers also ferment at lower temperatures than top fermented beers, this is not the entire explanation.
Top-fermented beers, which are most special beers, are fermented for 5 to 6 days at quite a high temperature, between 18 and 22 °C. The yeast then rises to the top of the tank and forms a thick layer on the beer. These beers are known as top fermented or high fermented beers because the yeast rises to the top.
The opposite applies to low-fermented beers, such as pilsners. After a fermentation period of 8 to 10 days at a lower temperature (10 to 12 °C), the yeast falls to the bottom of the fermentation tank. This is also known as under fermentation.
Temperature has nothing to do about whether the yeast rises to the top or falls to the bottom. This is due to the yeast culture used. Incidentally, the yeast culture also plays an important role in determining the beer’s aroma.