It's used to change the 'contents' of this brush (e.g. volumetric light makes it behave as water when entities going though it). You don't really need to use this.
Flags
2: "Use angles": If set, the func_illusionary will use its angles.
Notes
Even if non-solid, this entity still adds clipnodes. Set "ZHLT Disable Clipping" to 'Always non-solid' to prevent that or add zhlt_noclip key with value of 1.
