Looking for online file loudness analysis?
Does this online meter transfer data to the server?
No, all audio data is being used locally.
What mode does this meter have?
It has the ITU-R BS.1770-4, or dB A-Weighted mode.
How accurate is this meter compared to other professional SPL meters?
Analyzing from the microphone will not give you accurate SPL-Loudness values since it is limited to the hardware on which it is being used. If you want to know the exact SPL-Loudness level please use professional hardware made for that use. However, if you want to compare the perceived loudness of two different sounds this meter will do a great job.