Solids are the binding elements between the active parts of an arm.
They simply describe the distance from one rotor to the other.


If you have a setup like this:

+ is a rotor
= is a simple armor block

Assume that to the right you got positive X direction
Assume to the bottom you have positive Y direction

This would result in SolidSG( 5, 2, 0 ) since you have to go from the middle of the first rotor 5 blocks to the right to reach the second rotor. Also you have to go 2 blocks down.

Then to get from the second rotor to the third it is SolidSG( 4, -1, 0) because you go 4 to the right and one up.

So you pretty much place solid between every rotating part in your arm. Interestingly pistons themself count as solids as well, counting the length of it in blocks when it is completely retracted.