When no rotation is set, where do model point to?

I setup a little sandcastle [here].(Cesium Sandcastle)

There is one thing I do not understand about models.

Based on sandcastle exemple, Cesium represent axis as follow :
RED axis is forward,
GREEN is left,
BLUE is top.

Now, most of thing in cesium are always pointing to north., but for transform in models, the FORWARD is pointing EAST.

It’s a bit wierd that for model I always have to remove 90degree, or end up having problem in the previews.

Is it something that is documented somewhere ? or I am misunderstading something ?