Both terms are used to describe changes in the cervix. However, they are used in different situations. “Squamous intraepithelial lesion (SIL) is used to describe Pap test results. “Squamous” refers to the type of cells that make up the tissue that covers the cervix. SIL is not a diagnosis of precancer or cancer. The Pap test is a screening test. It cannot tell exactly how severe the changes are in cervical cells. A cervical biopsy is needed to find out whether precancer or cancer actually is present.
Cervical intraepithelial lesion (CIN) is used to report cervical biopsy results. CIN describes the actual changes in cervical cells. CIN is graded as 1, 2, or 3. CIN 1 is used for mild (low-grade) changes in the cells that usually go away on their own without treatment. CIN 2 is used for moderate changes. CIN 3 is used for more severe (high-grade) changes. Moderate and high-grade changes can progress to cancer. For this reason, they may be described as “precancer.” ACOG
Note: Even though the above separation exists, some cytologists still read pap smears and mention CIN because they say that you can look at the morphology of the cells and tell. Having said that, the above division clearly explains the difference between these two terms.
Reference
https://www.acog.org/Patients/FAQs/Abnormal-Cervical-Cancer-Screening-Test-Results#table1