Does it look like a red-green-blue 3-legged symbol? If so, that's called the spin center (not a c-system). It's used as a center of spinning of the 3D model. By default, it's located at the center of geometry of the model (not the center of mass). If you imagine a box enveloping the model, it would be at the center of that. You can toggle its display on and off fron an icon in the top toolbar.