What's the difference between Mask Layer and another one(the one with the crop icon) technically in this case?
The another one can be a pixel layer.
In that case, they seems to be no difference each other.
I've been using them without knowing the difference whole time.
But I like to know the difference to explain the layer system to other users.
Vector masking is cropping, not clipping. When you use a vector shape to mask another object, the cropping/masking shape becomes completely invisible. When you clip an object to a vector shape you create a ‘clipview’ object and any part of the clipping shape which is outside of the clipview area remains visible.
Part of the confusion around the terminology is probably explained by the fact that Serif uses the same definitions as Corel uses but some people are familiar with Adobe’s reference