Color is just one characteristic that goes into defining a standard. As our quality control systems evolved so did our tolerances with regard to other glass properties. Each product was soon being judged for surface texture, mix proportion, light transmission and other qualities. When a glass fell outside the tolerance range, it also received a T-Glass classification. So T-Glass came to encompass any glass we make that, for one reason or another, falls outside the QC limitations for it's standard.
Structural tolerances are a different matter altogether.
These are glass problems that translate into poor cutability, sheet warp, or structural weakness. Glass is not classified T-Glass for these reasons. So if you buy T-Glass, you can depend on it cutting just as well as any other Spectrum product. T-Glass is often a fine buy and a good way to save money on glass. The most common problem people encounter is needing to match it in the future.