RAK12019 is composed of an ambient light sensor (ALS) and a UV index sensor (UVS) module. When we use the UV index sensor module, we can see that the example code prints out two parameters, Uvi Data and Uvs Data, which come from the ltr.getUVI() and ltr.readUVS() functions, respectively.
It is not entirely clear what the specific meanings of Uvi Data and Uvs Data are. Based on the underlying code, Uvi is a calculated value of ultraviolet light data obtained from a formula, with units of uw/cm2, while Uvs is a value read from a register. Could you please provide a more specific explanation of the meanings of the Uvi Data and Uvs Data parameters?
I checked the datasheet of the LTR390. Maybe paragraph 7.2 answers your question.
For a more specific explanation, you might need to ask LiteON. We are selling the sensor, we are not UV light measurement experts nor do we know how the sensor works internally.