Standard Parasitic Exchange Format

Standard Parasitic Exchange Format (SPEF) is an IEEE standard for representing parasitic data of wires in a chip in ASCII format. Non-ideal wires have parasitic resistance and capacitance that are captured by SPEF. These wires also have inductance that is not included in SPEF. SPEF is used for delay calculation and ensuring signal integrity of a chip which eventually determines its speed of operation.

Source: Wikipedia — Standard Parasitic Exchange Format (CC BY-SA 4.0)