The hydrologic model network oftentimes is composed of several hydrologic model files representing the breaking-up of the total watershed into sub-watersheds that link together along major watercourses.
Because the hydrologic model network is developed according to a nodal numbering specification (namely, nodes numbers increase in the downstream direction; hint: try numbering nodes in increments of say 5 in order to allow future additions of nodes to the network), the topology of the network is described by the node numbers themselves. This property is extensively used throughout the SIMS module set in order to develop other networks for hydraulics models, costing models, pollution models, floodplains, sediment transport models, and so forth.
This BUTTON connects the numerous branches developed in each hydrology model file to the appropriate branch in neighboring files according to the topology properties associated with the node numbering layout set up by YOU. Once finished, the entire watershed is setup for processing by the selected hydrologic model.