The >0.3 measurement is the number of particles larger than 0.3 microns in 0.1L of air.
It is an extra measure we provide for people who want to know more about the exact particle counts for research or testing purposes. In general, we recommend looking at the AQI or the PM2.5 concentration.
What is the safe indoor limit for 0.3 PM particles?
Particle counts are typically used in a few specific industries, such as filtration and pharmaceuticals, where users are looking to measure the exact number of particles at varying sizes. Because particle counts are not widely used, there are not many accepted standards for a "safe level". This is also one of the reasons we suggest looking at the AQI or the PM2.5 concentration reading to understand air quality.