Is there a description for this somewhere? What does it do? How do I use it?

This is just a coremod. The units you want to control are in "Total Automization". Then you can use the Selectiontool from this mod to select the units you produced to move like in an RTS.

So does this contain the selection tool only? And if so, could it be used to control the default tank and gun buggy if I were to build multiples? Or does this only work with the units in the mod?

It contains the unit selection tool and deployer selection tool.

It only controls units, since tank and car and not units, they cannot be controlled.

Make tanks be units with selection tool?

No. Tanks are not units. If you want units that looks like tanks, install a mod that add units that looks like Tank.

This mod is only the feature of selecting units and giving commands to them.

FactorioCraft is getting close :)