That is math-speak for: How much you can trust this data.
Need to deep dive? Open the Smaart v9 manual to Chapter 6: "The Measurement Menu" for granular control over FFT size, windowing types (Blackman-Harris vs. Rectangular), and averaging modes. smaart 9 manual better
The Smaart 9 manual is a remarkable feat of technical writing for what it is: a complete specification of software behavior. But asking the manual to teach you how to tune a PA is like asking a dictionary to teach you how to write a poem. That is math-speak for: How much you can trust this data