This property is only used with subnetwork-based traces (SubnetworkTracer, UpstreamTracer, etc.). If DomainNetwork represents a ArcGIS.Core.Data.UtilityNetwork.TierDefinition.Partitioned network, this property is optional. If not null, the Trace routines will perform an additional check to validate that the starting points and barriers belong to this tier. If DomainNetwork represents a ArcGIS.Core.Data.UtilityNetwork.TierDefinition.Hierarchical network, this property is required. Since rows in hierarchical networks can belong to multiple tiers, this property tells the subnetwork tracer which tier to use for tracing.

Reference

TraceConfiguration Class
TraceConfiguration Members