On SKCropNode class reference, some examples to specify a mask are given.
Here they are:
This means a crop node can use simple masks derived from a piece of artwork, but it can also use more sophisticated masks. For example, here are some ways you might specify a mask:
An untextured sprite that limits content to a rectangular portion of the scene.
A textured sprite is a precise per-pixel mask. But consider also the benefits of a nonuniformly scaled texture. You could use a nonuniformly scaled texture to create a mask for a resizable user-interface element (such as a health bar) and then fill the masked area with dynamic content.
A collection of nodes can dynamically generate a complex mask that changes each time the frame is rendered.
The second example introduce nonuniformly scaled texture: what's the meaning of this?
This does not help me to understand this second example!
A non-uniformly scaled texture is a texture that is applied to a sprite with xScale != yScale.