Explaining the unexplained rooting terminology? LG G4 A rooting instruction post says I can root with the following models:

H811 10H
VS986 11A
H815 10C

Though I have an H815 I am unable to grasp what the '10C' part means.
I have thoroughly checked the back of the phone and there is no mention of any similar code, at least not one that is constant.

What does it mean?

Another similar rooting instruction post mentions '20a' and '20b' but doesn't say whether a version of 20a can be rooted with '20b' version or vice versa.

Can I assume that there are 100's of versions of the H815 variant of the LG G4?

Or does this '20a', '20b' just mean iterations of the same rooted stock android version?
Those are just build numbers of stock ROMs you can see in Settings - About Phone. Letters usually indicate minor updates and numbers major updates. By major it doesn't necessarily mean Android version bump - it could patch a number of important exploits, often including ones crucial to rooting.

A tool will be released with the exploit suitable for a version and all versions below it. Therefore, as long as the tool claims to root that version, you can do a lower one just fine.