Merging layers lets you group them permanently. You can merge multiple layers into one layer, or merge layers with the canvas. For more information about merging layers with the canvas, see Dropping layers with the canvas. To merge special, plugin, shape and text layers, you must first convert them to default layers. For more information about converting layers to default layers, see To convert to a default layer. You can also merge all visible layers. For more information about showing and hiding layers, see Showing or hiding layers. When you merge layers, the file size of an image is decreased but you can no longer edit the layers independently.
The order of the layers in the Layers panel determines how overlapping content appears in the merged layer. Content in layers that are at the top of the list replaces overlapping content on the underlying layers.