When you open or place a Photoshop file that contains multiple layers, you can set the following options:
Layer sets that use the Dissolve blending mode as well as all layers in clipping masks get merged into single layers.
Adjustment layers and layers that use a Knockout option are merged with the underlying layers, as are layers that contain transparent pixels and use the Color Dodge, Color Burn, Difference, Linear Burn, Linear Dodge, Vivid Light, Linear Light, or Pin Light blending mode.
Layers that use layer effects may be merged; however, the specific merging behavior depends on the blending mode of the layer, the presence of transparent pixels, and the layer’s blending options.
Hidden layers that require merging are discarded.