Hello all. I am an undergraduate student and I am very new to the term 'PDK'. When I skimmed the documentation of sky130 under nfet_01v8 symbol the first file is a bins.csv file which I have attached here. In that file there are some W's and L's specified with bin numbers. What are these? Can someone help me in understanding the concept of binning? Thank you in advance. https://cs.opensource.google/skywater-pdk/sky130_fd_pr/+/main:cells/nfet_01v8/sky130_fd_pr__nfet_01v8.bins.csv