You can allow or prevent connections between dissimilar parts. In general, you do not want to connect dissimilar systems. For unique design situations, however, you might want to allow such a connection or be prompted on a case-by-case basis.
For example, you might be drafting a water supply layout where you define one system for cold water and another for hot water. During layout, rather than having a third system for the main supply that enters the building, you might draft the main supply pipe on the cold water system and connect to the hot water heater that is a part of the hot water system. In this case, you would want to connect the hot water and cold water systems. You could configure layout rules to either allow the connection automatically or prompt you before making the connection.
For a connection to be valid, the connected parts must be assigned to the same system group and the connector style rules must be met. System groups are configured for part system definitions, and they enable you to connect different part systems that are assigned to the same system group. For example, you can connect medium-pressure supply duct to low-pressure supply duct if both are assigned to the supply system group.
When a connection test fails, you can prevent a connection from being made. You can also allow a connection of dissimilar systems, or allow a connection when the system of the part being drawn is automatically changed to match the system to which it is connecting.