Do I extrude or subdivide when adding new bones?

Forgive the newbie question! I recently downloaded a 3D model, which was already rigged using an IK chain. For my needs, I need to add additional bones to my model. I recognize the steps to add additional bones, but am unsure if the best choice for me is to “subdivide” or “extrude.”

For example, my current model has four spine bones; SpineBase, Spine1, Spine2, and Spine3. My end result requires me to have eight spine bones; I will need to add Spine4, Spine5, Spine6, and Spine7. I assume my best bet is to select Spine3 in Edit Mode, then use it as the starting point to add additional bones. However, I am unclear if going with subdivide or extrude would be ideal.

Screenshot of model provided here;

Best for what? Ideal for what?