The best way I learned some of these things was just building simple guides and testing multiple duplicates at the same time, using different settings, to see what changed.
Component index is for when you have more than one of something. So if you have a 4-armed monster, then you might want to make the lower arms
Or look at the fingers in the default biped template. Instead of “index”, “middle”, “pinky”, etc. you have finger_L0, finger_L1, finger_L2, etc.
Keep in mind that if you change the index, it changes the name of the controls, and you’ll have to edit your synoptic picker to match any new names.
Parent joint index lets you override where in the hierarchy something will get parented. As far as I know, -1 just means “the default first one”. I think if you set it to 0 it would usually do the same thing, maybe depending on your module.
For example, in this image I have parented 2
Control_01 modules under an arm root. Since the joints don’t exist until you build the rig, this is how you tell mGear which joint in the chain it should follow after it gets built.