What's a skybox?
In Second Life, a skybox is exactly what it sounds like: a building in the sky.
How high can a skybox be?
As shown in the limits list, the build height is 4,096m. However, the base of even a very high skybox will likely be placed below that (for example, 4,000m), allowing for room to be built upwards in case the owner wants to add an extra floor or other expansions.
Since avatars can fly in Second Life and placing objects in the air isn't affected by gravity (unless you choose to enable physics on them), making use of vertical space is very popular for several reasons, including:
- Privacy - It's less likely for both text and voice chat to be heard when it's hard to get to where you are. (Although IMs, instant messages, are an option anywhere.) New Residents and casual passerby are less likely to look thousands of meters in the air. Part of this is because there's a built-in fly height limit (also shown on the limits list), which can be bypassed with a scripted "flight assist" gadget, but is otherwise a default restriction.
"Skybox" in other 3D environments commonly means a simple construct to simulate skies.