COE, or Coefficient of Expansion, defines how much a material expands when heated and shrinks when cooled. It is one of the primary factors in determining whether two pieces of glass can be fused together without issues, also known as being compatible with each other.
Two pieces of glass with different COE values will not be compatible with each other, most likely resulting in cracks in the final piece. This is because the different glasses will expand and contract differently, causing stresses within the resulting work.
However, it's important to not automatically assume that two pieces of glass with the same COE value will be compatible with each other. There are other factors at play.
Officially, Bullseye Glass Co doesn't like to label their glass as having a particular COE value. They have taken this stance to try to avoid issues with people placing too much emphasis on COE being the only determining factor in compatibility.
Unofficially, Bullseye glass is widely recognised as having a COE of 90. But while Bullseye ensure compatibility among their different fusible glasses, they provide no such guarantee of the compatibility between their fusible glass and COE 90 glass from other manufacturers. Any fusing between glasses from different manufacturers is done so at the user's own risk.
For more information, check out Bullseye's TechNote on the subject of compatibility.