OPL vs. GML

Question: 

What is the difference between the OMRI Products List and the OMRI Generic Materials List?

The OMRI Products List and OMRI Canada Products List consist of individual brand name products that have been reviewed by OMRI and confirmed to be compliant with the National Organic Program or Canadian Organic Regime Standards, respectively.
The OMRI Generic Materials List (GML) describes whether different generic substances, such as mined minerals and synthetic micronutrients, comply with the National Organic Standards. The GML is comparable to the Permitted Substances Categories for Canada.
Brand name products on the OMRI Products Lists are listed with a specific use Class and Category corresponding to the Generic Materials List or Permitted Substances Categories.