Originally, SIM was the 2G application inside the Universal Integrated Circuit Card or UICC. People use the word SIM card to refer to UICC, which ETSI standardizes.SIM form factors relate to the physical dimensions of the SIM card, encompassing traditional sizes such as Full-Sized, Mini, Micro and Nano SIMs and have shrunk from the original plastic card 1FF. A nano-SIM can function as an eSIM, and an eSIM can exist in different form factors, such as MFF2.