Which of the following two directions provides the most beneficial results to a thriving business: integrating isolated systems and service "stacks" vertically, or horizontally?
Let's define this mumbo-jumbo first...
Vertical, in this process or technological context, usually refers to things which share some functional aspect. Maybe they're related by business function (finance, hiring, manufacturing, etc.), or by ownership (division, department, etc.).
Horizontal, in this same context, usually refers to things which normally do not rub against each other during any "normal" operational context. For example, an HR employee database, and maybe the warehouse temperature monitoring and control system.
But what happens when you need to know how many things have passed in and out of the warehouse, which were in some way, any way, influenced by a particular division, department, group, budget code, or individual employee? What about tapping the facilities systems to identify the impact of storing and shipping X items of product Y for employee Z?
What about identifying how much money was moved around related to a specific purchase order? Sounds easy enough. Let's say you find a particular P.O. and it ordered a truckload of XYZ100 contraptions. The items had to be stored in special facilities with strict temperature and humidity controls. They required special trucks to move them, and the people that handle them had to be certified to a very specific policy, usually resulting in higher hourly rates for their time. So, now, I ask again: how much TOTAL money was moved around by this P.O.?
If you said "vertical", you might want to reconsider.