You're diagramming your network. Typically network engineers like using a tree layout - core devices at the top, flowing down to aggregation devices, down to access devices, down to the end devices. Hence traffic to the North was to the core, and out, traffic SOuth was to the end devices.

Typically in a campus you would see traffic going from the end devices up to the core, and then out of the core, either to datacenters/machine rooms, or out to the internet.

In a datacenter, historically, you had a few servers that talked to each other, connected to the same "Access" switch (commonly refered to as the top-of-rack or end-of-row), and then almost all the traffic for those servers also went "north" to the core, with a much smaller amount going south. Almost all the traffic was from clients out in the corporate network, down to their specific set of services.

However over time, end users represent a smaller and smaller portion of what an application does. More systems integrate with more systems - pulling in data from many other systems, doing analysis, backup, etc. etc. This is the east - west traffic, that flows between things in the same tree diagram. East-west traffic is by far the largest throughput in a modern DC.

When the traffic was mostly north-south, network engineers secured the traffic at the edge of the DC - where the DC joined the core/internet. Now the traffic is between servers that are sitting in the same rack/row/room/DC, securing it in the same way just doesn't work.