Instancing is a feature of IRender nXt and AccuRender nXt for Revit where trees or components which are repeated several times in a drawing are generated once, and then repeated as Instances for additional uses.
Instancing saves memory when rendering because it only has to store the definition of the Component or Tree once, but takes longer to renderer because the instance has to be rotated, offset and scaled each time it is accessed. Components which are instanced render about 50% slower than components which are generated individually for each instance.
There is a setting on the IRender Special Tab which determines how many times an identical component must be reused before it is instanced.
In the current version of IRender nXt Instancing is only implemented for trees.