An ad-hoc network is when you open a network on an entity. This will show the entity in a network with all the directly linked neighbors, except when you open a network for a document or multiple items. In the latter two cases, only the selected entities open in a network. More nodes can be added to such networks using different techniques. See Adding Nodes To Network and Adding Neighbors.
Select an entity in the margin area, a manager, the project explorer or any browser, right-click and select Show in Network.
The nodes are initially placed using a default layout procedure, but can be rearranged manually or using any of the other layout procedures. See Layout And Routing.
Below you see an ad-hoc network of a folder containing three categories and their subcodes.