Horizontal split (Extended) vertical split (extended)

Scale out (also scale horizontally) scales horizontally, scaling out
Scale up (that is, scale vertically) scales vertically, scaling up
Scale Out,scale Up,scale in is actually a concept of architecture that can be used on storage, on a database, and on a network.
The simple analogy of scale out and scale up helps us understand:
Scale out, for example: We add a new machine to the original web and mail system.
Scale up, for example: We add CPU, memory to the original machine.

Horizontal split (Extended) vertical split (extended)