Blue triangle in camera helpers

I might have missed this, but in the camera helpers what is this blue triangle for please? The screenshot is from a DirectionalLight shadow helper.

It’s the “up” direction of the light.

https://threejs.org/docs/#api/en/core/Object3D.DefaultUp

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I still do not get I am afraid. In the sceenshot for example the light is shinning from the top-right to the bottom-left. Is this is the up direction (ie top-right to bottom-left) please?