When you start using Affinity Photo or Designer, I think the most confusing part of the application is clipping of layers as there are 2 variants. Clipping involves positioning one layer inside another layer.
Clipping layers
These are also known as Masking layers as they are mainly used for masking. I do prefer to use the Clipping layer term as these type of layers are not only used for masking. In fact a mask is nothing else then clipping out part of a layer.
When you add a mask to a layer, the default behavior will be that it will be added as a clipping layer.
This also applies for a live Filter
In the layers panel the clipping layers are always shown beside layer icon. To add another layer as a clipping layer, you need to drag it on top of the layer icon. Pixel/Image or Vector layers added a clipping layer will act as a mask.
Child Layers
The other way of applying clipping is using Child layers. With child layer, the parent layer becomes the new boundaries for the child layer. Any areas of the child layer which lie outside the parent object's layer are clipped (hidden).
To create a child layer, you can drop the layer not the icon of the layer but on the text of layer in the layers panel.The layer will be moved inside the selected layer and has become a child layer. The child layer is not clipped inside the parent and replace the pixels in the parent layer. When you collapse the parent layer, child layers will not be shown.
Btw, another way of adding a child layer is by using the Arrange menu. You can use the move inside menu item from the Arrange menu to make the current layer a clipped child layer of the layer below.
To summarize
A clipping layer acts a clip to mask from the applied layer and a (clipped) child layer overwrites the parent with as restriction that it gets clipped within the boundaries of the parent (the parent acts like a mask). Both are child layers in the sense that they are part of another layer. The correct terminology would probably we a CLIPPING child and a CLIPPED child.
I also posted a video on the subject in the past (based on version 1). Most of the differences mentioned in the video, do not apply to version 2.1 anymore.