Explanation about read values

[Bishamon1987]

Heey there,
I was looking at the code and didnt really understand some read values. What are the units.
For instance:
Speed get its value from 'avg_distance_cmps'
Instantaneous pace gets its value from a calculation (500 * 100) / value of 'avg_distance_cmps'.

What is 'avg_distance_cmps' what unit does it use? m/s, mil/h etc?
What are the numbers 500 and 100 and why not use 50000 instead of calculating it first?

Maybe make a list of values and what they mean with units and explanation of the math behind it if it is used?