A layer mask is used to reveal a portion of a layer while the rest of the layer remains hidden. This means that you can use a mask layer to 'delete' areas of a layer that you don't want.
In Affinity Designer, two types of masking are possible: pixel masking and vector masking.
Masks are applied to designs as a separate layer, allowing them to be freely edited and moved. Mask layers affect any object below them within the same parent layer. They can also be clipped to individual objects so that only that object is affected.
The thumbnail of the target object changes to indicate that a mask and crop have been applied.
The mask of the object is applied in a "crop to top object" operation.