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 as a separate layer, allowing them to be freely edited and moved. Mask layers affect any layer below them within the same parent layer. They can also be clipped to individual objects so that only that object is affected.
Do one of the following:
On the Layers Studio, choose New Mask Layer.
The added mask will hide areas outside a selection (if a selection is in place) or display the entire layer (if no selection is in place).
On the Layers Studio, choose New Empty Mask Layer.
The added mask will hide the entire layer regardless of any selection in place.