A plumbing system definition is typically named for the plumbing system for which it will be used, such as domestic hot water or sanitary sewer. The settings for a system definition control many of the display properties of a plumbing system. For example, a layer key style is assigned to a plumbing system definition, which dictates the layer, color, and linetype for the plumbing system.
In the software, plumbing runs that have different systems cannot connect to each other without changing the system for the connecting run. The exception is when the 2 system definitions comprise the same system group. System groups allow different systems to connect to each other.