A network node server list indicates the network nodes that can act as servers for an end node. The network node server list is not required, but using it allows you to control which network node is selected by an end node.
An end node becomes aware of adjacent network nodes that can act as its network node server through normal, spontaneous network flows. By default, an end node establishes a CP-CP session with the first network node of which it becomes aware, and this network node acts as the end node's server. There is no predictability in this scheme because it depends on timing.
You might want to control which network node is selected by an end node. For example, you might want to exclude a particular network node from network node server responsibilities; acting as a server does involve some overhead, such as originating search requests or executing domain broadcasts. You might also want to isolate particular end nodes from certain network nodes for security reasons.
See Network node server list for more details.