When glass changes temperature, it expands and contracts. The rate at which glass changes size is called the coefficient of thermal expansion. If you fuse two glass pieces together and one changes size faster or slower than the other, the fused piece may crack--even several months after fusing.
When different glasses have a close enough coefficient of expansion to fuse successfully, they’re called fusing compatible. Buy glass labeled fusing compatible. Or fuse glass that has been cut from the same sheet, which guarantees compatibility.
Fusing glass is rated with a coefficient of expansion number (i.e. COE 90 or COE 96). The pieces that you fuse together should have the same COE number. Carefully label glass storage containers with the COE number to avoid mixing different types of glass.