Generally, Meas Input Rate will be used as the scaling source so that audio signal frequency measurements follow any changes in sample rate from the source.
If the actual measured value is unstable, you may enter the nominal incoming sample rate in the Rate Ref field and select DIO Rate Ref for stable audio frequency scaling.
The Output Rate selection can be used to measure the frequency- shifting effects of digital processors and sample rate converters.
The Status Bits selection refers frequency measurements to the nominal, standard sample rate (if in fact the sample rate is actually encoded into the status bits) and will be independent of any moment-to- moment variations in the actual received sample rate.