A seasonal factor measures the percentage amount that on average, monthly production is above or below normal. A seasonal factor of 120 states that the month in question will usually be 20% above an average month's production level. Similarly, a seasonal factor of 100 implies an average month, while a factor of 80 implies a month that is typically 20% below average.
The not seasonally adjusted (NSA) data for auto assemblies are obtained from Ward's Communications, the weekly-basis seasonal factors (SFW), which contain adjustments for holiday and length-of-month effects as well as normal seasonal variation, and the number of production days, or weekdays in a month, are applied to MVAAUTOASN using the formula found here.
Board of Governors of the Federal Reserve System (US), Weekday-Basis Seasonal Factors: Auto Production [G17MVSFWAUTOS], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/G17MVSFWAUTOS, December 8, 2021.