This extracted circuit is needed for various purposes including circuit simulation, static timing analysis, signal integrity, power analysis and optimization, and logic to layout comparison.
Each of these functions require a slightly different representation of the circuit, resulting in the need for multiple layout extractions.
In this case, by associating the extracted circuit with the layout and the logic network, the user can cross-reference any point in the circuit to its equivalent points in the logic and layout (cross-probing).
These parts are inter-related since various device extractions can change the connectivity of the circuit, e.g., resistors (whether designed or parasitic) convert single nets into multiple electrical nodes.
This summary was derived, with permission, from Volume II, Chapter 22, Layout Extraction, by William Kao, Chi-Yuan Lo, Mark Basel, Raminderpal Singh, Peter Spink, and Lou Scheffer.