https://doi.org/10.1351/goldbook.C00835
Carbon fibres type IM (intermediate modulus) are related to carbon fibres type HT because of the comparable values of tensile strength, but are characterized by greater stiffness (Young's modulus up to approximately $\pu{35\%}$ of the theoretical \(C_{11}\) value).
Note: The tensile modulus (Young's modulus) varies between ca. \(275\) and \(350\ \rm{GPa}\), but the disposition of the boundaries is somewhat arbitrary. The relatively high ratio of tensile strength to tensile modulus, typically above \(1 \times 10^{-2}\), in carbon fibres type IM, in spite of an increase of Young's modulus, requires a further increase of strength, which is achievable by a significant reduction of the monofilament diameter down to about \(5\ \unicode[Times]{x3BC}\rm{m}\). Such small filament diameters are typical of carbon fibres type IM.