Hello aleksandar95,

How do sigma b and sigma c depend on the distance between points?

I already posted a link to the documentation. The equation reads

$\sigma_{\delta h} = \sqrt{\sigma_a^2 + \sigma_b^2 d + (\sigma_c d)^2}$

where d is the distance.

For example, if the leveling distances are between 100 and 200 meters, what should be the value of sigma b?

The value you expect for this type of a-priori uncertainty.

For instance, if the instrument’s accuracy is 1mm/km, then sigma b is 1. Is this correct?

It is a good starting value to define this component of the stochastic model, yes.

I am most interested in the value of the sigma a parameter. What should its value be and what does it depend on?

Like σb and σc, σa depends on the assumed uncertainty of your measurement procedure.

Does sigma a represent an a priori standard deviation or not?

Again, like σb and σc, σa is an a-priori uncertainty, too.

If it refers to the uncertainty of a single measurement, does that mean it is the expected error value?

No. It is the assumed distance-independent part of the combined uncertainty of the measurement procedure. However, in case of σb=σc=0, σa defines the assumed a-priori uncertainty of a single observation δh, cf. the equation above.

Kind regards
Micha