What does the definition actually mean?
The droplet size was in the early watermist stage seen as an important tool to categorize watermist since people thought that droplet size and the firefighting performance was correlated. Therefore the oldest standards and codes differentiated watermist systems depending on the system droplet size, and demanded that droplet sizes where measured.
This is not the case in any updated codes and standards, since it is long proven that only full scale fire testing, and not droplet sizes, will show whether a system can cope with a certain risk or not.
The reason people thought that there was a correlation was because watermist showed remarkable results in machinery enclosures where the small water droplets easily could evaporate and become inert gas (water steam) due to the heat created by the large fires and extinguish any fires as a gas system fast and efficient.
Empirical test evidence however show that a mixture of many different size droplets can do the same in enclosures and provide even better results in deep seated Class A fires. For the same reason watermist systems shall always be chosen based on positive testing results and approvals, and not droplet sizes alone.