I find this odd and I’m willing to understand it, but I’m wondering why objects that would otherwise be referred to parts or components in a model are designated as tools. What are these tools used for? What’s the theory behind this organization conceit?
It’s basically a legacy term. ZBrush started as an illustration program with 3D elements, so (as the page Doug links to says) any “tool” could be used on the ZBrush canvas. The illustration aspect is still there - there are 2.5D tools - but as ZBrush has moved more towards 3D, they are not used much.