It makes some sense in comparison to horizontal when looking at the structure of an organisation: you can either slice it at seniority level, so all the workers together in one layer, all the managers, all the upper management, all the executives and so on.

Or, you can slice it vertically, into departments, with all the levels of seniority together grouped by what they're actually doing. You could call them departments, you could call them markets, but those terms are already taken.