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Jointed Model Skeleton (JMS) is the intermediate geometry file format used to import models into Halo's tag formats. Despite the name, it is used for all types of models even if they aren't rigged, such as level geometry.
It is the target format when exporting from 3D editors like Blender and 3ds Max (using their respective addons), or after conversion from FBX using H1A Tool. Using Tool, a JMS file is then compiled into one of Halo's geometry tags: