There is no difference, all of these designations are for Top Grade sheets except for Select Grade.

Fully Stocked
The solid surface material is currently in production and should be available on a continuous basis.

Overstocked
Excess full sheet material from manufacturers, distributors, and/or fabricators.

Discontinued
Solid surface material no longer in production by the manufacturer. This material comes from manufacturers, distributors, and/or fabricators and specific colors are not available on a continuous basis.

Partial Sheets
Also known as surplus, drop-off, or fall-off, remnants are Top Grade solid surface pieces of varying sizes left over from fabrication projects from warehouses and fabrication shops all over the country. (Note: Remnants are sold as is. These pieces are typically in "like new condition" but may have scratches, scrapes and chipped corners).

Select Grade Select grade material does not qualify for the Top Grade designation because it is a slight bit off the color specification desired by the manufacturer either because of the base color or the decorative flecks embeded in the base color.