If I put 4 cubes in a scene in a modelling program (not touching each other), and exported it to .x for example, that could be considered a "model" or a "mesh" or a "polygon mesh". The one or more objects in the model are treated by dbp as individual "Limbs" (as they are actually individual meshes in modelling app as well). If you rotated the Object in dbp all four cubes would move as if they are one object. If you rotated a Limb, only that limb would move as if it was its own model.
Limbs are nothing more than what makes up the model object, but is a dbp term really, not so much a modelling term.
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