In a typical circuit board design, many of the signal trace runs are very similar. As an example, consider traces where the logic drivers are the same, the trace length is between certain limits, the rise time is fixed, there are no stubs, the lines are series terminated, and the traces are all stripline with defined trace parameters. Traces that have this fixed set of attributes are said to belong to one net. In a typical design, there can be dozens of distinct nets. Computer programs are available that can verify that traces assigned to a net meet the requirements of that net.
The designer must first decide on IC types, board material, trace widths, etc. There are tools that will analyze transmission line performance for a set of choices. If a signal does not arrive within the expected time envelope then the trace may have to be assigned to a different or new net list. If the design reaches a snag and the design must be changed then all net lists must be re-examined.