property Opacity: Byte; //read and write

When layer opacity = 0 (zero), then that layer will be fully transparent. When opacity = 255 (the default), then opacity will be solely dependant on the opacity of each pixel's alpha channel. Otherwise, each pixel in the contained image will be displayed according to the following formula:
pixel opacity = pixel alpha * layer opacity div 255.

Note: adjusting the layer's opacity will not alter the image it contains, just how that image will be displayed when merged by TLayeredImage32.GetMergedImage.

See Also