Information presented below this line is outdated syntax or information used for Custom Stuff 1. It will not work with Custom Stuff 2.
The opacity level of a block determines how much light can pass through a block. The higher opacity is set to, the more light that is blocked. Any whole number between 0 and 255 can be used for the opacity attribute. By default, opacity is set to 255, causing all light to be blocked. If you want to make any block that does not fill an entire block, it is a good idea to set the opacity attribute to 10 or less to let light through. This is important for any block type that is partially transparent. In other words, any type other than 'normal' or 'simple'. The reason for needing to set the opacity to a low value for these block types is that when determining the light value within a block, Minecraft uses the light value on the block directly below it. If all light is blocked, a visual 'bug" will be caused.
//blocks all light from passing through
//allows enough light to fix opacity issues
//allows all light through the block