The highest temperature at which the material can reasonably be used. It is further subdivided by what limiting factor is reached at that point.
The decomposition limit concerns the point where the chemical composition of the material begins to be permanently altered. The autoignition limit concerns the point where the material will begin to outright burn. These two types lend themselves to specific, objective values.
For mechanical and corrosion limits, there is not standard method for measuring or calculating the value — instead, it tends to be a broadly agreed-upon industry convention. The factors that go into picking a value depend on the types of applications that the material is commonly used for. Such factors include what % loss of properties is tolerable, to what extent creep is important, and what time scales of high temperature exposure are being considered. This makes it harder to compare dissimilar materials.