Most potteries had to use a letter and number system to keep track of the many shapes and treatments that were available at any given time. Each company had their own system. For example, the Taylor Smith & Taylor Company would use a number for a specific treatment regardless of the shape. The Edwin M. Knowles China Company would do the same, but add a suffix to denote the edge color as in 501-E1 for pattern 501 with gold edge or 501-E0 for pattern 501 for no trim at all. The W. S. George Pottery Co. had a very complex system and after many years ot trying to figure it out, I'm still not 100% sure on how it worked.
The Homer Laughlin China Company numbering system included a prefix with either the shape initial or a customer's initials. "WW" stood for Woolworth's, "MW" for Montgomery Wards, etc. A specific decal could have multiple treatment numbers depending on the amount of shapes, different edge treatments, and in a few cases, the grade of ware.
These pictures, taken by author Joanne Jasper in the 1990s, are of treatments used on HLC's Century shape and are prefixed with a "C" except for the three MS patterns at the bottom. Decal books like these have a wealth of information. As you can see there are notes on the grade of ware; Scheme, RK (for Run of Kiln), and thirds. There are notes on how the corresponding hollowware should be decorated as well as type of glaze (vellum or ivory). In many cases, there is a reference to the originating decal. For example, C-107 and C-205 were originally used with the Orleans shape as O-19. C-200 uses a decal from the Jade shape (J-6).
At the bottom of the page are photos taken at the factory of Century sample plates with their treatment numbers.
The following represent only a portion of the many decorations used on the Century shape. For more on the Century shape, see this page.