Tiers demarcate a logical level within a network. For example, in an electric distribution network, there may be Subtransmission, Medium Voltage and Low Voltage tiers.
The numeric rank of the tier. It is possible for several tiers to have the same numeric rank. For example, in an electric distribution network, Radial Medium Voltage and Radial Multifeed Medium Voltage tiers would have the same rank. When doing a trace between a source tier and a target tier, the source and target tiers define an upper and lower tier value range for a trace. The trace will stop if it encounters a tier whose rank is outside the range specified by the source and target tiers.