Devices and lines are both modeling equipment. Generally, per ISO/ANSI standard C.84 breaks equipment in to three overall groups High Voltage, Medium Voltage and Low Voltage. High Voltage could be divided further into Extra High and Ultra High.
Second is for cartographic reasons, mainly performance of scale suppression. Generally speaking, you would want High Voltage equipment to be visible at scales larger than you would want Medium or Low Voltage equipment visible.
Third, attributes and domains assigned to attributes. Domains can only be assigned at the Subtype or Asset Group level. Also, attribute aliases and visibility can only be set at the Subtype or Asset Group level.
Many people come from a CIM background. CIM is not a good cartographic model. It's a good electrical model, but a really bad cartographic model.
John Alsup
jalsup@esri.com