As you are a good lover for UNI-T equipment (which I personally find too a really price efficient alternative), we cannot left Fluke's team work without any merit.

They give an explanation of what Accuracy Percentage and Digits mean for them (Why digital multimeter accuracy and precision matter):

Accuracy may also include a specified amount of digits (counts) added to the basic accuracy rating. For example, an accuracy of ±(2%+2) means that a reading of 100.0 V on the multimeter can be from 97.8 V to 102.2 V. Use of a digital multimeter with higher accuracy allows for a great number of applications.

As you can read from your equipment datasheet:

• Selected Range: \$0-60 Hz\$

• Resolution: \$0.001 Hz\$

• Accuracy: \$\pm 0.02%\$ +8 Digits (% Reading + Digits)

Hence, for a 50.000 Hz signal, with normally 5 significant digits for the UN181A, we should expect a +-50*0.0002 Hz=+-0.01 Hz error from the % Reading

Plus, the digits accuracy.  \$\pm1\$ digit means if you have a 50.000 Hz reading, you could expect an error between 49.999 and 50.001. Thus, 8 digits=+-00.008 Hz error when reading 50.000 Hz, with a total error of $$ \pm0.01 + 0.008 Hz = \pm0.018 Hz $$

And that is!. We cannot infer anything else outside from this range. Electrical gaussian noise, temperature drifts, sensor drifts, internal compensations, etc. are all in this range and every other further extrapolation, without any specific advice from UNI-T is a side conclusion.

As you are a good lover for UNI-T equipment (which I personally find too a really price efficient alternative), we cannot left Fluke's team work without any merit.

They give an explanation of what Accuracy Percentage and Digits mean for them (Fluke.com, Why digital multimeter accuracy and precision matter):

Accuracy may also include a specified amount of digits (counts) added to the basic accuracy rating. For example, an accuracy of \$\pm(2\%+2)\$ means that a reading of \$100.0 V\$ on the multimeter can be from \$97.8 V\$ to \$102.2 V\$. Use of a digital multimeter with higher accuracy allows for a great number of applications.

As you can read from your equipment datasheet:

• Selected Range: \$0-60 Hz\$

• Resolution: \$0.001 Hz\$

• Accuracy: \$\pm 0.02%\$ +8 Digits (% Reading + Digits)

Hence, for a \$50.000 Hz\$ signal, with normally 5 significant digits for the UN181A, we should expect a \$\pm50\cdot0.0002 Hz=\pm0.01 Hz\$ error from the % Reading.

The typical format should be 50.000, so the digits accuracy should be taken from this representation of significative digits. 

\$\pm1\$ digit means if you have a 50.000 reading, you could expect a reading between 49.999 and 50.001. Thus, 8 digits error means an \$\pm 0.008\$ Hz error when reading 50.000.

Then, you should expect a total error of: $$ \pm(0.01 + 0.008) Hz = \pm0.018 Hz. $$

And that is!. We cannot infer anything else outside from this range. Electrical gaussian noise, temperature drifts, sensor drifts, internal compensations, etc. are all in this range and every other further extrapolation, without any specific advice from UNI-T is a side conclusion.

As you could realize, every sigma interpretation would assume a distribution, for which, in this case, do not have information to conclude. Typically we shouldn't too, since the drifts would be more significative.