Consider the 6-millimeter rule: When heated to full fusing temperatures, most Bullseye sheets will naturally assume a thickness of about 6 millimeters unless constrained by dams or molds. For projects like plates and tiles, you may want to start with a base of two layers of Standard 3 mm sheet glass. A base thinner than two Standard layers, when fired, will tend to shrink or draw in to assume a 6 millimeter thickness. To learn more about this basic kilnforming principle, see TechNote 5: Volume & Bubble Control.
Given the 6-millimeter rule, you can consider when you might want to use Thin glass. Some common uses include adding design elements on top of base glass (thick design elements may dilute color in the base or distort edges) and stacking multiple layers of color within a piece.