The IP proportions (typically shown in the first column of the relevant tables in the G.17 release) are estimates of the industries' relative contributions to overall growth in the following year. For example, the relative importance weight of the motor vehicles and parts industry is about 6 percent. If output in this industry increased 10 percent in a month, then this gain would boost growth in total IP by 6/10 percentage point (0.06 x 10% = 0.6%).
NAICS = 3261
Source Code: RIW.G3261.S
Board of Governors of the Federal Reserve System (US), Relative Importance Weights (Contribution to the Total Industrial Production Index): Manufacturing: Non-Durable Goods: Plastics Product (NAICS = 3261) [RIWG3261S], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/RIWG3261S, April 15, 2021.