What does the guiding principle for using layer masks in photo editing state?

The guiding principle for using layer masks in photo editing is that black conceals, while white reveals. This principle is foundational in non-destructive editing, allowing photographers and digital artists to manipulate visibility without permanently altering the original image.

When a layer mask is applied, areas painted with black on the mask will hide the corresponding parts of the layer, effectively making them invisible. Conversely, areas painted with white will show the content of the layer, allowing those portions of the image to remain visible. This duality gives users the flexibility to refine their edits by switching between revealing and concealing specific elements of their layers, facilitating precise control over the editing process.

This principle enables various creative techniques, such as softening edges, blending multiple images, and selectively enhancing parts of an image while keeping others hidden, without the risk of losing any original data.