A unique values renderer where symbols are assigned to features based on an unique attribute values.
Remarks
I LookupSymbol is used as a faster alternative to IFeatureRenderer::SymbolByFeature when querying for a features symbol during labeling. While SymbolByFeature hands out clones of a symbol, ISymbolLookup provides a framework where references to symbols are handed out and modifications of the symbol are reset. Your renderer should implement this interface to make labeling perform efficiently.