Ultem 9085, the most common ultem resin used for AM, is a blend of PEI-PC, as seen here
Ultem is a trade name for PEI alloys made by Sabic and 9085, used in filaments made by both Stratasys and 3dXtech as the two most visible suppliers are both made with this same alloy. It is used for high temperature resistance and strength and needs to be printed at upwards of 300C in a contained environment.
Source-Intern at Made in Space.