Usually the terms vertical and horizontal are used to refer to use among different industries, not describing use within a single company. Think of different industries (healthcare, banking, etc.) each as a silo filled with many different companies.

A vertical product would target just one silo. It doesn't matter that not everyone at each company in that silo would use the product, what is important is that the product is only meant to be used by companies in that silo. An example would be software to x-rays. This would be used by some people at the companies in the healthcare silo, but would be of no use to anyone in the banking silo. The product would be tailored to exactly the task that needs to be completed.

A horizontal product would target many or all of the silos. The product would have no specific focus, but would be intended to be generally useful. An example of this would be Excel. Almost all companies in every silo would have someone who uses Excel. They may also use it for vastly different purposes requiring the software to be somewhat generic and the end user to figure out how to use the software to complete the task at hand.